Abrasive Grain With Main Surfaces and Subsidiary Surfaces

ABSTRACT

An abrasive grain includes a surface having at least a first face with a first outline, and at least one second face with a second outline. The first outline does not contain any vertices, but the second outline contains at least one vertex. The abrasive grain may include a ceramic material, especially polycrystalline α-Al 2 O 3 .

This application is a divisional application of copending U.S. patentapplication Ser. No. 14/417,886, which was filed on Jan. 28, 2015 and isa 35 U.S.C. §371 National Stage Application of PCT/EP2013/066105, filedon Jul. 31, 2013, which claims the benefit of priority to (i) patentapplication no. DE 10 2012 213 629.0, filed on Aug. 2, 2012 in Germany,(ii) patent application no. DE 10 2012 213 632.0, filed on Aug. 2, 2012in Germany, (iii) patent application no. EP 12178931.7 filed on Aug. 2,2012 in Europe, and (iv) patent application EP 12178937.4, filed on Aug.2, 2012 in Europe. The disclosures of each of the above-identifiedpatent applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present disclosure relates to abrasive grains with a defined form,collectives of abrasive grains, methods for producing abrasive grains,casting tools, abrasive articles with abrasive grains, methods forproducing abrasive articles and also methods for grinding a surface withan abrasive article.

BACKGROUND

Abrasive grains, in particular ceramic abrasive grains, with a definedform and size have been known for some time.

U.S. Pat. No. 5,201,916 discloses inter alia flat abrasive grains withfor example a trianglular, rectangular or circular form. These abrasivegrains are produced from a dispersion which contains particles that canbe converted into α-alumina and a liquid with a volatile component. Thedispersion is poured into a casting mold, which has a planar base areaand depressions, the forms of which are complementary to the desiredforms of the abrasive grains. Subsequently, part of the volatilecomponent is removed, and so a precursor with the desired form isproduced. The precursor is then removed from the mold, calcined andfinally sintered so as to produce the finished abrasive grain.

The abrasive grains produced by this method have two opposite mainfaces, which have substantially the same geometrical form. The abrasivegrains are given a longer lifetime, since small pieces continually breakoff from the abrasive grains during grinding, and so new cuttingsurfaces are obtained. The abrasive grains thereby sharpen themselves.It is presumed that, in the case of the abrasive grains with a base areain the form of a triangle, in particular an equilateral triangle, withelectrostatic scattering approximately one to two thirds become orientedin such a way that one vertex faces away from the backings, while otherabrasive grains orient themselves in such a way that the vertex facesthe backing.

In an alternative method, described in EP 615 816, elongate, rod-shapedprecursors are first produced by means of extrusion, and then dividedinto individual abrasive grains. The rod-shaped abrasive grains mayconsequently have for example the form of a cylinder or prism.

In WO 2009/085841 there is a description of a further method ofproduction in which the precursor is dried in the casting mold underconditions that lead to fracturing of the precursor. The shards at leastpartially have surfaces and edges that are complementary to thecorresponding surfaces and edges of the casting mold, and therefore havethe angle defined by the casting mold. These surfaces and edges providean enhanced cutting capability. On the other hand, the further surfacesand edges produced by the fracturing are irregular.

WO 2010/077495 discloses abrasive grains which contain openings passingright through or not passing right through or have a dish-like form.Methods of production for such abrasive grains are also described there.Further abrasive grains with undefined openings are disclosed in WO2010/077518. WO 2010/077491 is likewise concerned with abrasive grainswith a dish-like form.

WO 2010/077519 discloses abrasive grains that have two opposite mainfaces and side faces that extend between them and are inclined inrelation to the main faces. The various side faces of an abrasive grainmay be inclined at different angles in relation to the main faces.

The document WO 2011/068724 likewise shows abrasive grains that have abase side and a vertex and also inclined side faces extending inbetween. Similar forms of abrasive grain are also described in WO2011/109188.

The document WO 2010/077509 is concerned with abrasive grains that havea surface with a multiplicity of grooves. These grooves are producedwith the aid of complementary ridges on the underside of the castingmold.

WO 2011/068714 shows pyramidal abrasive grains with aparallelogram-shaped, in particular rhomboidal, base area, a kite-shapedbase area and a superelliptical base area.

WO 2011/139562 discloses abrasive grains in the form of tetrahedrons andmodifications thereof. For example, the side faces may be concavely orconvexly formed, the corners of the tetrahedron may be truncated, or theedges may be curve-shaped.

The abrasive grains described in WO 2012/018903 include two or moreplate-shaped portions, which are arranged at an angle in relation to oneanother.

In the case of the method described in WO 2012/061016, firstly anabrasive formation is produced, containing abrasive grain precursorsthat are connected to one another by way of frangible webs. Aftersintering, the abrasive grains are separated from one another by thewebs being severed.

Alternatively, abrasive grains of a defined form may also be produced bya screen printing process. This is described for example by WO 96/12776.In this case, a dimensionally stable dispersion is passed throughopenings with a defined form onto a transporting belt and subsequentlycured. The openings may for example be contained in a movable endlessbelt.

A further development of the screen printing process is disclosed in WO2011/087649. In the case of this process, the dispersion is forcedthrough the openings in the endless belt by means of a differentialpressure. Given a suitable choice of the viscosity of the dispersion,with this process it is possible to produce abrasive grains of a crosssection that tapers from a first main side to a second, opposite mainside.

In WO 2012/061033 there is a description of methods for producingabrasive grains of a defined form with the aid of laser radiation.Moreover, further special forms of abrasive grains are disclosed. Forexample, the abrasive grains may include a main element and at leastthree rod-shaped elements extending from it. In particular, the abrasivegrain may have the form of a cross, an uppercase letter “T”, a star or alowercase Greek letter “λ”.

It is generally assumed that abrasive grains with a defined form haveimproved properties from several aspects: if the abrasive grains have adefined form and size right from the beginning of their production,there is no need for a subsequent sorting step, by which the abrasivegrains would otherwise have to be divided into different size fractions.Moreover, the forms and sizes also remain virtually unchanged betweendifferent production batches, which makes the abrasive propertiesreproducible very well. Furthermore, the abrasive grains may for exampleprovide an increased overall removal, have a longer lifetime, produce anenhanced surface quality of the surface worked or provide abetter-reproducible grinding result.

Nevertheless, the abrasive grains known from the prior art have a seriesof disadvantages. For example, many of the known abrasive grains cannotbe arranged on the abrasive material backing in a very space-saving way.Moreover, many of the known abrasive grains cannot be anchoredsufficiently in a binder applied to an abrasive material backing.

SUMMARY

It is therefore an object of the present disclosure to overcome at leastpartially the disadvantages of the prior art. In particular, therefore,it is intended to provide an abrasive grain that is formed in such a waythat a multiplicity of such abrasive grains can be arranged on anabrasive material backing in a space-saving way. Moreover, the abrasivegrain should be formed in such a way that it can be anchored as securelyas possible in a make coat applied to an abrasive material backing. Inadvantageous configurations, the abrasive grain should at the same timebe of such a kind that improved chip formation and/or increased removalof material can be achieved. Moreover, in preferred embodiments, theabrasive grain should be configured in such a way that, even with theaid of mechanical scattering, it can with the greatest probability bearranged on a backing in an advantageous orientation. There is anadvantageous orientation for example whenever corners and/or edges ofthe abrasive grain face away from the backing and therefore face asurface to be worked.

This object is achieved by an abrasive grain that has a surface with atleast a first face and at least a second face. The first face has afirst outer contour, which is bounded by the first face; the second facehas a second outer contour, which is bounded by the second face.According to the disclosure, the first outer contour does not include acorner, while the second outer contour does however include at least onecorner.

Here and hereinafter, a face is understood as meaning a contiguoustwo-dimensional part of the surface of the abrasive grain that consistsof points at which a well-defined, imaginary tangential plane can beplaced against the abrasive grain. Such a face may be planar or curved,in particular concavely curved or convexly curved. It may also have atleast one planar portion and at least one curved portion, which go overinto one another without an edge lying in between.

A corner on the second outer contour is understood here and hereinafteras meaning a point of the second outer contour at which the direction ofthe tangent to the second outer contour runs discontinuously. Accordingto the disclosure, the first outer contour does not include a corner.This therefore means that the direction of the tangent that can beplaced against the points of the first outer contour runs continuouslyalong the entire first outer contour. The first outer contour mayinclude at least one straight portion and/or at least one curvedportion, in particular at least one convexly curved portion and/or atleast one concavely curved portion. At the points at which adjacentportions touch one another, the direction of the tangent should howeverrun continuously. For example, the first face may be circular orelliptical.

The second outer contour includes a least one corner, as defined above.The second outer contour may also include at least one straight portionand/or at least one curved portion, in particular at least one convexlycurved portion and/or at least one concavely curved portion.

If such an abrasive grain according to the disclosure lies with thefirst face on an abrasive material backing, and in particular the firstface is embedded in a make coat applied there, the abrasive grain isanchored particularly securely. This is so because then there are nocorners of the abrasive grain there that could otherwise produce cracksin the make coat when working a surface, which could lead to a breakoutof the abrasive grain. Moreover, abrasive grains with corners on thefirst outer contour could hinder one another if, when scattering on anabrasive material backing, they are arranged adjacently. On account ofthe absence of corners on the first outer contour, the abrasive grainscan consequently in many cases be placed with a greater density on abacking. The corners of the second outer contour that are kept at adistance from the first outer contour, and therefore also from thebacking and the make coat, and in particular face away from it, can onthe other hand provide an abrasive effect.

The first face is preferably substantially planar. This allowsparticularly secure fixing on an abrasive material backing when theabrasive grain comes to lie with the first face on this backing.

Alternatively or in addition, the second face may be substantiallyplanar.

Expediently, the first face and the second face lie opposite one anotherand are arranged in relation to one another at an angle that is lessthan 30°, preferably less than 20°, more preferably less than 10°, andis particularly preferably 0°. In particular if the first face and thesecond face are planar, it is advantageous if the abrasive grain extendscompletely between the two planes that are defined by the first face andthe second face.

If an angle as mentioned above is formed between the first face and thesecond face, in particular an angle of 0°, and the abrasive grain lieswith its first face on an abrasive material backing, the second facealso extends at the angle mentioned in relation to this backing, inparticular therefore parallel to the backing. This allows a well-definedabrasive effect to be achieved.

In geometrically simple embodiments, the second face is formed by apolygon. The corners of the polygon then form the corners of the secondouter contour. Both these corners and the edges extending between themprovide an improved chip-forming effect and increase the removal ofmaterial from a surface worked. The polygon may be convex oralternatively have at least one reflex corner. It may for example be atriangle, in particular an isosceles triangle or even an equilateraltriangle, a quadrangle, in particular a parallelogram or even arectangle or even a square, a pentagon, in particular a regularpentagon, or a hexagon, in particular a regular hexagon. Regularpolygons are preferred, since they have a high degree of symmetry, whichleads to uniform grinding properties.

In advantageous configurations, the perpendicular projection of thesecond face onto the first face lies completely within the convexenvelope of the first face. This convex envelope is the smallest convexsurface that includes the first face. The projection mentionedpreferably even lies within the first face itself. As a result, theabrasive grain is particularly stable with respect to the tilting forcesduring grinding when it lies with the first face on an abrasive materialbacking.

Particularly preferably, the abrasive grain tapers along a directionrunning perpendicularly from the first face. This therefore means thatthe perpendicular projection of each first cross-sectional areaextending parallel to the first face and at a first distance from itlies completely within each second cross-sectional area extendingparallel to the first face and at a second, smaller distance from it.This allows particularly good anchorage in a make coat applied to abacking, since the abrasive grain has a kind of “standing foot”.Moreover, this has the effect that, even with mechanical scattering, theabrasive grain with great probability comes to lie with the first faceon the backing.

Between the first face and the second face, a lateral area may beformed. It is favorable if the lines of intersection of the lateral areawith at least one sectional plane that extends perpendicularly to thefirst face and perpendicularly to the second face are at least partiallyconcave. In many exemplary embodiments, this likewise has the effect ofproducing a kind of “standing foot”, which allows improved anchorage ina make coat. The property mentioned advantageously applies to everysectional plane that extends perpendicularly to the first face andperpendicularly to the second face. It is likewise advantageous if thesectional lines are completely concave. Moreover, it is expedient if thesectional lines are strictly concave.

The form and the size of the abrasive grain can be determined forinstance with the aid of a microscope. The abrasive grain according tothe disclosure may have a size in the entire range of sizes that is alsocustomary for conventional abrasive grains. Usually, abrasive grainswith larger sizes lead to a greater removal of material from a workedsurface than smaller abrasive grains. For example, the abrasive grainmay have a size in the range from 100 μm to 2000 μm. This size can bedetermined experimentally with the aid of a microscope. It is understoodas the diameter of an enveloping circle of the first face, that is tosay as the smallest diameter of a circle which encloses the first face.If the abrasive grain has multiple first faces without a corner, thesize of the abrasive grain is the greatest of the diameters of all thesefaces.

The form described above of the abrasive grain is an idealization.However, the disclosed abrasive grain also comprises abrasive grainsthat deviate from this idealized form within the limits of productiontolerances. Possible deviations from the idealized form may be due toone or more of the following causes:

cavities or small bubbles on account of trapped air and/or other gasesin a dispersion from which the abrasive grains are produced;

missing corners and/or edges that are produced due to incomplete fillingof a casting mold and/or during removal of a precursor of the abrasivegrain from a casting mold;

sunken side faces and/or edges that are produced due to shrinkage duringthe removal of part of the volatile components of the dispersion; inparticular, sunken faces that are produced from the upper free surfaceof the dispersion, which is not in contact with the casting mold;

instances of flaking that are caused by a drying and/or sinteringprocess;

broken-off corners and/or edges that are produced by transporting and/orduring further processing of the abrasive grains as bulk material.

The deviations from the idealization do not necessarily have to lead todisadvantageous properties of the abrasive grain. For example,broken-off corner and/or edges can also have the effect that furthercutting edges in comparison with the idealization are produced, and mayeven positively influence the abrasive effect.

The disclosure also comprises in particular abrasive grains of which theform only coincides substantially with the idealized form. For example,the first or the second face are understood as substantially planar ifthey are curved and have radii of curvature that are at least twice,preferably at least five times, particularly preferably at least tentimes, the size defined above of the abrasive grain. Furthermore, apoint of the second outer contour is regarded as a corner if thereactually is a curvature there, the curvature of which however is at most10%, preferably at most 5%, particularly preferably at most 2%, of thesize of the abrasive grain. Similarly, portions of the first or thesecond outer contour are regarded as straight if they are at leastpartially or even completely curve-shaped and have a radius of curvaturethat is at least twice, preferably at least five times, particularlypreferably at least ten times, the size of the abrasive grain. Inparticular, an outer contour is also regarded as a polygon if it is madeup of such substantially straight portions and corners.

However, the abrasive grain preferably has an idealized formed asdescribed above.

The abrasive grain may for example contain or consist of a ceramicmaterial, in particular a polycrystalline ceramic material. The abrasivegrain preferably contains aluminum oxide, particularly preferablyα-Al₂O₃.

Alternatively or in addition, the abrasive grain may also contain atleast one further metal oxide, such as for instance sodium oxide,magnesium oxide, iron oxide, silicon oxide, calcium oxide, zirconiumoxide, yttrium oxide, zinc oxide, cobalt oxide, nickel oxide, hafniumoxide, chromium oxide, praseodymium oxide, samarium oxide, ytterbiumoxide, neodymium oxide, lanthanum oxide, gadolinium oxide, cerium oxide,dysprosium oxide, erbium oxide, lutetium oxide, titanium oxide,manganese oxide or any desired combinations thereof.

Many of these metal oxides originate from impurities in the starting rawmaterials, such as for example in aluminum oxide. With sufficientlysmall fractions in the abrasive grain, such impurities do not howeverhave any adverse influence on the production and application of theabrasive grain. Some of the impurities mentioned may even have apositive effect on the abrasive grain.

Fractions of zirconium oxide or yttrium oxide may for example originatefrom grinding balls that can be used in a grinding step in theproduction of the abrasive grains. Fractions of iron oxide may originatefrom a grinding vessel that is used in such a grinding step.

Likewise alternatively or in addition, the abrasive grain may containfurther hard materials, such as for example silicon carbide.

Furthermore, the abrasive grain may contain at least one breakdownproduct of a dispersant described in more detail below that was used inthe production of the abrasive grains. In addition, the abrasive grainmay comprise at least one nucleating agent or breakdown product thereofthat was used in the production of the abrasive grains. The nucleatingagent may be for example the magnesium oxide already mentioned above.

Moreover, the abrasive grain may also contain at least one of thefurther substances described in EP 615 816 A1.

The ingredients mentioned can be determined with the aid of chemicalanalysis methods known per se.

The abrasive grain may contain or consist of a structure having one ormore different phases. A first phase may consist of aluminum oxide,particularly preferably of α-Al₂O₃. A second phase may consist of one ormore of the aforementioned further metal oxides and/or further hardsubstances.

The proportion of aluminum oxide, in particular of α-Al₂O₃, in theabrasive grain may be for example at least 25% by weight, preferably atleast 50% by weight, more preferably at least 70% by weight,particularly preferably at least 95% by weight.

The abrasive grain may have a coating which covers only part of thesurface, in particular only one or more edges and/or only one of anumber of flat regions of the surface. The coating may for instance be aferromagnetic or paramagnetic coating. Such a partial coating of thesurface with a ferromagnetic or paramagnetic material makes it possibleto align the abrasive grain in a given direction in a magnetic fieldapplied during the scattering operation. Alternatively, it may also be acoating of a material with increased thermal conductivity or a coatingthat makes enhanced adhesion of the abrasive grain on the abrasivematerial backing possible.

A further aspect of the disclosure relates to a collective of abrasivegrains. A collective of abrasive grains is understood here andhereinafter as meaning a coherent collection of abrasive grains. Forexample, this may be a collection of abrasive grains that are containedin a container and are stored and/or transported as such, for example ina sack.

Such a collection of abrasive grains can be used to produce an abrasivearticle. The entirety of all the abrasive grains present in an abrasivearticle is also regarded as a collective of abrasive grains.

Preferably, the collective of abrasive grains includes at least 20% byweight, preferably at least 50% by weight, particularly preferably atleast 90% by weight, of abrasive grains according to the disclosure, asdescribed above. The other abrasive grains included in the collectivemay likewise have a defined form, but one that differs from the formaccording to the disclosure, or they may not have a defined form sincethey are for example fractured abrasive grains. These other abrasivegrains included in the collective are also referred to as “supportinggrains”.

It is conceivable and within the scope of the disclosure that theabrasive grains according to the disclosure included in the collectiveare formed differently from one another. For example, the collective ofabrasive grains may include a first fraction of abrasive grains of afirst embodiment according to the disclosure and also a second fractionof abrasive grains of a second embodiment according to the disclosurethat is different from the first embodiment according to the disclosure.In particular, the abrasive grains of the first embodiment according tothe disclosure may differ from the abrasive grains of the secondembodiment according to the disclosure in their size and/or in theirform.

The collective of abrasive grains may consist exclusively of identicalabrasive grains according to the disclosure; in particular, thecollective then has a size distribution in the form of points.

The collective of abrasive grains may substantially have a sizedistribution that conforms to a size standard customary in the abrasivesindustry, for example the American National Standards Institute (ANSI),the Standards of the Federation of European Producers of Abrasives(FEPA) or the Japanese Industrial Standard (JIS). For example, thecollective of the abrasive grains may substantially have a grain size ofP12, P16, P20, P24, P30, P36, P40, P50, P60, P80, P100, P120, P150,P180, P220, P240, P280, P320, P360, P400, P500, P600, P800, P1000,P1200, P1500, P2000, P2500, P3000 or P5000 according to the FEPAstandard. In this context, “substantially” have a size distributionmeans that at least 90% by weight, preferably at least 95% by weight,more preferably at least 99% by weight and particularly preferably allof the abrasive grains in the collective of abrasive grains meet thisstandard.

As already described above, it is also conceivable that the collectiveincludes at least two different fractions of abrasive grains accordingto the disclosure and/or at least one fraction of abrasive grains notaccording to the disclosure. Each of these fractions may itself have asize distribution that respectively conforms to one of theaforementioned size standards customary in the abrasives industry.

An abrasive grain according to the disclosure or a collective ofabrasive grains according to the disclosure can for example be producedby the following method known from U.S. Pat. No. 5,201,916:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain.

Before and/or during the preparation of the dispersion in step a), theraw materials, in particular α-alumina particles and/or particles thatcan be converted into α-alumina, may be ground. This may be performedfor example with the aid of a ball mill, in particular with the aid of aplanetary ball mill.

The dispersion may contain at least one dispersant. Such a dispersantfacilitates the formation of the dispersion and increases its stability,for example by forming around the individual grains layers that preventclumping. The dispersant may be for example a polymer. Generally, thedispersant breaks down at the latest during the sintering in step g).

A casting tool may be used for producing the abrasive grains accordingto the disclosure, the casting tool comprising at least one castingmold, which has at least one depression with a respective surface, thesurface being complementary to the form of at least part of the surfaceof the abrasive grain.

There are preferably a multiplicity of depressions in the casting mold,and so a multiplicity of abrasive grains can be cast by one castingoperation.

In a further development of the method known from U.S. Pat. No.5,201,916, the aforementioned casting mold may be only part of amultipart casting tool that additionally comprises at least one moldingelement, in particular a further casting mold or die element, withwhich, in addition to the surface molded in the first casting mold, atleast part of the remaining surface of the abrasive grain can be molded.This is meaningful in particular whenever the abrasive grain to beproduced has undercuts.

For example, die elements that are guided into the filled depressions inthe first casting mold may be provided. Alternatively, it is alsoconceivable that, in the optional step c), not all of the dispersionthat stands above the upper side of the casting mold is wiped off, butinstead a layer of a suitable thickness is left over the upper side ofthe casting mold. The dispersion in step c) should be dimensionallystable enough for this. For example, a dispersion that is alreadydimensionally stable enough may be introduced in step b), or thedispersion may be transformed into a state that is dimensionally stableenough between steps b) and c). In the case of this variant, the dieelements do not necessarily have to enter the depression; instead,shaping may be performed with the aid of the die elements in the part ofthe dispersion that stands above the mold.

The die elements preferably do not close off the depressions in thecasting mold completely, and so the volatile component of the dispersioncan escape.

More complicated forms of abrasive grains can be formed in casting moldsthat can be assembled, in a way similar to in the injection-moldingprocess. For this purpose, at least one casting mold has at least onefilling opening, through which the dispersion can be introduced into thedepressions.

The casting mold and/or the die element may for example contain orconsist of silicone. The depressions may have an open top surface,through which the dispersion can be introduced. The depressions in thecasting mold have in each case a surface of a form that is complementaryto the form of at least part of the surface of the desired abrasivegrain. Preferably, the depressions in the casting mold have in each casea surface of a form that is complementary to the form of the part of thesurface of the abrasive grain that does not form the first face of theabrasive grain or does not form the second face of the abrasive grain.The first face or the second face may then be produced from the upperfree surface of the dispersion, which is not in contact with the castingmold. On account of possible shrinkage of the dispersion during step d),it is possible that the first face or the second face is not completelyplanar, but has a slight curvature.

The precursor produced in step d) should preferably be mechanicallystable enough to be further processible as bulk material in thesubsequent steps. The optional calcining in step f) is advantageous, oreven required, in particular whenever the dispersion contains a numberof different raw materials and a phase transformation is required.

Another aspect of the disclosure relates to an abrasive article thatcontains a collective of abrasive grains as described above. Inparticular, it is thus possible for at least 20% by weight, preferablyat least 50% by weight, particularly preferably at least 90% by weight,of all the abrasive grains of the abrasive article to be formed asabrasive grains according to the disclosure, as described above. Theother abrasive grains may likewise have a defined form, but one whichdiffers from the form according to the disclosure, or they may not havea defined form.

The abrasive article may be for example a coated abrasive article(coated abrasive), a nonwoven abrasive article, a bonded abrasivearticle (bonded abrasive) or an abrasive brush.

A coated abrasive article includes a backing, in particular a flexiblebacking, such as for example paper, vulcanized fiber, a film, a textilematerial, a foam or multilayer combinations thereof. The abrasive grainscan be secured to the backing with the aid of a base binder (“makecoat”). The make coat and the abrasive grains may be covered with a topbinder (“size coat”). Optionally, it is also possible for there to beabove the size coat mentioned a second top binder (“supersize coat”).

All binders known per se, for example of synthetic resin, such as forinstance a phenolic resin, an epoxide, a urea resin, a melamine resin oran unsaturated polyester resin, may be used as the make coat, size coatand supersize coat. The size coat and/or supersize coat may also containfurther customary active ingredients and/or fillers.

The abrasive article may take different product forms, for example thatof an abrasive disk or that of an abrasive belt.

The disclosure also comprises a method for producing an abrasive articleaccording to the disclosure, as described above. The method includes astep in which a collective of abrasive grains is fixed on and/or in asubstrate, in particular by means of a binder. The substrate may be forinstance a backing, in particular a flexible backing, of a coatedabrasive article, a nonwoven material of a nonwoven abrasive, a matrixof a bonded abrasive or bristles of an abrasive brush. In the case of acoated abrasive article, the make coat and/or the abrasive grains and/orthe size coat and/or the supersize coat can be applied by a method knownper se. For example, the abrasive grains can be appliedelectrostatically or mechanically (i.e. gravimetrically). In particularin the case of the advantageous embodiments described above, even withmechanical scattering, a large proportion of the abrasive grains areoriented such that the first face comes to lie on a backing. This makesit possible to dispense with more complex electrostatic scattering.

Furthermore, the disclosure is also directed to a method for grinding asurface with an abrasive article as described above. The surface may bein particular a painted surface. In the case of a painted surface,abrasive grains with sizes of 500 μm or less are particularly suitable.

The object is also achieved by an abrasive grain with a surface thatincludes at least one main face with an outer contour that has at leastseven corners.

Here and hereinafter, a face, and in particular a main face, isunderstood as meaning a contiguous two-dimensional part of the surfaceof the abrasive grain, which consists of points at which a well-definedimaginary tangential plane can in each case be placed on the abrasivegrain. Such a face may be planar or curved, in particular concavelycurved or convexly curved; it may also have at least one planar portionand at least one curved portion, these portions merging into one anotherwithout an edge lying in between.

The outer contour of the main face may include at least one straightportion and/or at least one curved portion, in particular at least oneconvexly curved portion and/or at least one concavely curved portion.

According to the disclosure, the main face has an outer contour with atleast seven corners. In very many exemplary embodiments, the abrasivegrain then also has a multiplicity of edges, whereby a high abrasiveeffect can be achieved. For example, the outer contour of the main faceincludes at least seven edges, which connect the at least seven cornersto one another. In addition, the abrasive grain may include for each ofthe corners of the outer contour of the main face at least one furtheredge, which connects this corner to a point outside the main face.

The number of corners of the main face is at least seven, or may be atleast eight, at least nine, at least ten, at least eleven, at leasttwelve, at least 13, at least 14, at least 15, at least 16, at least 17,at least 18, at least 19 or at least 20. It may be precisely seven,precisely eight, precisely nine, precisely ten, precisely eleven,precisely twelve, precisely 13, precisely 14, precisely 15, precisely16, precisely 17, precisely 18, precisely 19 or precisely 20.

With particular advantage, the main face is substantially planar. Thisis so because then the abrasive grain can be fixed particularly securelyto a planar abrasive material backing when it comes to lie with the mainface on this backing.

In some geometrically simple configurations, the abrasive grain may beformed as a cone with a base area of which the outer contour has atleast seven corners or a greater number of corners as specified above. Acone is understood here generally as meaning a geometrical body that isbounded by a substantially planar base area and a multiplicity ofsubstantially straight generating lines, each point of the outer contourof the base area being connected to a common point of the cone (thevertex of the cone) by one of the generating lines in each case, thevertex of the cone lying outside the plane defined by the base area. Inparticular, the cone may be a pyramid in which the base area is formedby a polygon with at least seven corners. From each corner of the basearea of such a pyramid, an edge of the abrasive grain extends to thevertex of the pyramid and can provide a cutting effect. As analternative to this, however, it is also conceivable that the base areaof the cone has at least one curved portion or even exclusively curvedportions, as long as the number of corners that are arranged betweenthese curved portions is at least seven.

In other geometrically simple configurations, the abrasive grain may beformed as a frustocone with a base area of which the outer contour hasat least seven corners or a greater number of corners as specifiedabove. By analogy with the general definition above, a frustocone isdefined as part of a cone in which the vertex of the cone has beenremoved by a substantially planar section. The top area produced by thissection likewise has at least seven corners. In particular, thefrustocone may be a frustopyramid with a base area that is formed by apolygon with at least seven corners. From each corner of the base areaof such a frustopyramid, an edge of the abrasive grain extends to acorresponding corner of the top area and can provide a cutting effect.Moreover, the top area of the frustopyramid includes the same number ofedges as the base area, which leads to a further increase in the cuttingeffect.

In further geometrically simple configurations, the abrasive grain maybe formed as a cylinder with a base area of which the outer contour hasat least seven corners or a greater number of corners as specifiedabove. A cylinder is understood here and hereinafter as meaninggenerally a body that is bounded by a base area and a top area and alsoby a lateral area. The base area and the top area are in this casepreferably substantially planar and likewise preferably substantiallyparallel to one another. The lateral area is formed by an array ofsubstantially mutually parallel lines. If these lines run substantiallyperpendicularly to the base area and the top area, the result is astraight cylinder. However, skewed cylinders are likewise conceivableand are within the scope of the disclosure. The base area of thecylinder may be for example a polygon with at least seven corners; inthis case, the abrasive grain is therefore formed as a prism.

It is likewise conceivable that the abrasive grain is formed as anantiprism with a base area of which the outer contour has at least sevencorners or a greater number of corners as specified above. An antiprismis bounded by precisely two congruent n-edged polygons and 2n triangles,in particular equilateral triangles. At a corner, there is always ajunction of one n-edged polygon and three triangles. The total number ofedges of the antiprism is 4n, that is n edges on the base area, n edgeson the opposite top area and 2n edges that run between the base area andthe top area. For these embodiments, according to the disclosure n is atleast 7, and so the total number of edges is at least 28.

The abrasive grain may also have the form of a twisted cone, inparticular a twisted pyramid, a twisted frustocone, in particular atwisted frustopyramid, a twisted cylinder or a twisted antiprism. Thismeans that the sectional areas of the twisted body parallel to the basearea are turned by an angle with respect to the non-twisted body, thisangle depending on the distance of the sectional area from the basearea. For instance, this angle may depend linearly on the distance ofthe sectional area from the base area. For example, a twisted cylinderwith a base area in the form of a regular heptagon may haveapproximately the form of a bolt with a seven-start thread.

If the main face of the abrasive grain is substantially planar, theabrasive grain may be assigned a height that is then understood as thegreatest perpendicular distance of a point of the abrasive grain fromthe main face. The main face may also be assigned an area diameter. Thisis the smallest diameter of a circle that includes the main face; it canconsequently be understood as the diameter of an enveloping circle ofthe main face. The ratio between the height and the area diameterpreferably lies in the range from 0.8 to 1.4, more preferably from 0.9to 1.2, particularly preferably from 0.95 to 1.05. It is advantageousfor mechanical scattering if the ratio mentioned lies in the lowersubrange of this range, since, with mechanical scattering, the abrasivegrain is then preferably oriented with its main face on an abrasivematerial backing. In terms of advantageous chip-forming behavior,greater ratios within the range mentioned tend to be expedient, sincethe angle between the abrasive grain and a worked surface lies closer to90°.

In many geometrically simple embodiments, the base area is a polygonwith at least seven corners or a greater number of corners as specifiedabove. This is the case for example in the exemplary embodimentsdescribed above of a pyramid and a frustopyramid. The disclosure,however, comprises still further geometries with a polygonal base areathat are neither pyramids nor frustopyramids nor cylinders. The polygonmay be convex; this means that the internal angle at each corner is lessthan 180°. However, it is also conceivable and within the scope of thedisclosure that the polygon has at at least one corner an internal anglethat is greater than 180°, and so the polygon is not convex. Forexample, the polygon may have the form of a star. In some embodiments,the polygon is regular; this means that all of the edges of the polygonare equal in length and also the internal angles at all the corners areequal in size.

The form and the size of the abrasive grain can be determined forinstance with the aid of a microscope. The abrasive grain according tothe disclosure may have a size in the entire range of sizes that is alsocustomary for conventional abrasive grains. Usually, abrasive grainswith larger sizes lead to a greater removal of material from a workedsurface than smaller abrasive grains. For example, the abrasive grainmay have a size in the range from 100 μm to 2000 μm. This size can bedetermined experimentally with the aid of a microscope. It is understoodas the area diameter described above of the main face of which the outercontour has at least seven corners. If the abrasive grain includes morethan one main face of which the outer contour has at least sevencorners, the size of the abrasive grain is understood as the greatestarea diameter of all these main faces.

The form described above of the abrasive grain is an idealization.However, the disclosure also comprises abrasive grains that deviate fromthis idealized form within the limits of production tolerances. Possibledeviations from the idealized form may be due to one or more of thefollowing causes:

cavities or small bubbles on account of trapped air and/or other gasesin a dispersion from which the abrasive grains are produced;

missing corners and/or edges that are produced due to incomplete fillingof a casting mold and/or during removal of a precursor of the abrasivegrain from a casting mold;

sunken side faces and/or edges that are produced due to shrinkage duringthe removal of part of the volatile components of the dispersion; inparticular, sunken faces that are produced from the upper free surfaceof the dispersion, which is not in contact with the casting mold;

instances of flaking that are caused by a drying and/or sinteringprocess;

broken-off corners and/or edges that are produced by transporting and/orduring further processing of the abrasive grains as bulk material.

The deviations from the idealization do not necessarily have to lead todisadvantageous properties of the abrasive grain. For example,broken-off corner and/or edges can also have the effect that furthercutting edges in comparison with the idealization are produced, and mayeven positively influence the abrasive effect.

The disclosure also comprises in particular abrasive grains of which theform only coincides substantially with the idealized form. For example,part of the surface of the abrasive grain is also regarded as a (single)face if this part actually includes two or more contiguous parts of anarea which touch one another at an edge at which there is included aninternal angle that lies in the range from 160° to 200°, preferably from170° to 190°, particularly preferably from 175° to 185°. Furthermore, apoint of the surface is regarded as a corner if there actually is acurvature there, the radii of curvature of which however are at most10%, preferably at most 5%, particularly preferably at most 2%, of thesize defined above of the abrasive grain. Furthermore, the main face isalso understood as substantially planar if it is curved and has radii ofcurvature that are at least twice, preferably at least five times,particularly preferably at least ten times, the size defined above ofthe abrasive grain. Moreover, the form of the abrasive grain is alsoregarded as a cylinder, cone or frustocone if the generating lines areat least partially or even completely curved and have a radius ofcurvature that is at least twice, preferably at least five times,particularly preferably at least ten times, the size of the abrasivegrain.

However, the abrasive grain preferably has an idealized formed asdescribed above.

The abrasive grain may for example contain or consist of a ceramicmaterial, in particular a polycrystalline ceramic material. The abrasivegrain preferably contains aluminum oxide, particularly preferablyα-Al₂O₃.

Alternatively or in addition, the abrasive grain may also contain atleast one further metal oxide, such as for instance sodium oxide,magnesium oxide, iron oxide, silicon oxide, calcium oxide, zirconiumoxide, yttrium oxide, zinc oxide, cobalt oxide, nickel oxide, hafniumoxide, chromium oxide, praseodymium oxide, samarium oxide, ytterbiumoxide, neodymium oxide, lanthanum oxide, gadolinium oxide, cerium oxide,dysprosium oxide, erbium oxide, lutetium oxide, titanium oxide,manganese oxide or any desired combinations thereof.

Many of these metal oxides originate from impurities in the starting rawmaterials, such as for example in aluminum oxide. With sufficientlysmall fractions in the abrasive grain, such impurities do not howeverhave any adverse influence on the production and application of theabrasive grain. Some of the impurities mentioned may even have apositive effect on the abrasive grain.

Fractions of zirconium oxide or yttrium oxide may for example originatefrom grinding balls that can be used in a grinding step in theproduction of the abrasive grains. Fractions of iron oxide may originatefrom a grinding vessel that is used in such a grinding step.

Likewise alternatively or in addition, the abrasive grain may containfurther hard materials, such as for example silicon carbide.

Furthermore, the abrasive grain may contain at least one breakdownproduct of a dispersant described in more detail below that was used inthe production of the abrasive grains. In addition, the abrasive grainmay comprise at least one nucleating agent or breakdown product thereofthat was used in the production of the abrasive grains. The nucleatingagent may be for example the magnesium oxide already mentioned above.

Moreover, the abrasive grain may also contain at least one of thefurther substances described in EP 615 816 A1.

The ingredients mentioned can be determined with the aid of chemicalanalysis methods known per se.

The abrasive grain may contain or consist of a structure having one ormore different phases. A first phase may consist of aluminum oxide,particularly preferably of α-Al₂O₃. A second phase may consist of one ormore of the aforementioned further metal oxides and/or further hardsubstances.

The proportion of aluminum oxide, in particular of α-Al₂O₃, in theabrasive grain may be for example at least 25% by weight, preferably atleast 50% by weight, more preferably at least 70% by weight,particularly preferably at least 95% by weight.

The abrasive grain may have a coating which covers only part of thesurface, in particular only one or more edges and/or only one of anumber of flat regions of the surface. The coating may for instance be aferromagnetic or paramagnetic coating. Such a partial coating of thesurface with a ferromagnetic or paramagnetic material makes it possibleto align the abrasive grain in a given direction in a magnetic fieldapplied during the scattering operation. Alternatively, it may also be acoating of a material with increased thermal conductivity or a coatingthat makes enhanced adhesion of the abrasive grain on the abrasivematerial backing possible.

A further aspect of the disclosure relates to a collective of abrasivegrains. A collective of abrasive grains is understood here andhereinafter as meaning a coherent collection of abrasive grains. Forexample, this may be a collection of abrasive grains that are containedin a container and are stored and/or transported as such, for example ina sack.

Such a collection of abrasive grains can be used to produce an abrasivearticle. The entirety of all the abrasive grains present in an abrasivearticle is also regarded as a collective of abrasive grains.

Preferably, the collective of abrasive grains includes at least 20% byweight, preferably at least 50% by weight, particularly preferably atleast 90% by weight, of abrasive grains according to the disclosure, asdescribed above. The other abrasive grains included in the collectivemay likewise have a defined form, but one that differs from the formaccording to the disclosure, or they may not have a defined form sincethey are for example fractured abrasive grains. These other abrasivegrains included in the collective are also referred to as “supportinggrains”.

It is conceivable and within the scope of the disclosure that theabrasive grains according to the disclosure included in the collectiveare formed differently from one another. For example, the collective ofabrasive grains may include a first fraction of abrasive grains of afirst embodiment according to the disclosure and also a second fractionof abrasive grains of a second embodiment according to the disclosurethat is different from the first embodiment according to the disclosure.In particular, the abrasive grains of the first embodiment according tothe disclosure may differ from the abrasive grains of the secondembodiment according to the disclosure in their size and/or in theirform.

The collective of abrasive grains may consist exclusively of identicalabrasive grains according to the disclosure; in particular, thecollective then has a size distribution in the form of points.

The collective of abrasive grains may substantially have a sizedistribution that conforms to a size standard customary in the abrasivesindustry, for example the American National Standards Institute (ANSI),the Standards of the Federation of European Producers of Abrasives(FEPA) or the Japanese Industrial Standard (JIS). For example, thecollective of the abrasive grains may substantially have a grain size ofP12, P16, P20, P24, P30, P36, P40, P50, P60, P80, P100, P120, P150,P180, P220, P240, P280, P320, P360, P400, P500, P600, P800, P1000,P1200, P1500, P2000, P2500, P3000 or P5000 according to the FEPAstandard. In this context, “substantially” have a size distributionmeans that at least 90% by weight, preferably at least 95% by weight,more preferably at least 99% by weight and particularly preferably allof the abrasive grains in the collective of abrasive grains meet thisstandard.

As already described above, it is also conceivable that the collectiveincludes at least two different fractions of abrasive grains accordingto the disclosure and/or at least one fraction of abrasive grains notaccording to the disclosure. Each of these fractions may itself have asize distribution that respectively conforms to one of theaforementioned size standards customary in the abrasives industry.

An abrasive grain according to the disclosure or a collective ofabrasive grains according to the disclosure can for example be producedby the following method known from U.S. Pat. No. 5,201,916:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain.

Before and/or during the preparation of the dispersion in step a), theraw materials, in particular α-alumina particles and/or particles thatcan be converted into α-alumina, may be ground. This may be performedfor example with the aid of a ball mill, in particular with the aid of aplanetary ball mill.

The dispersion may contain at least one dispersant. Such a dispersantfacilitates the formation of the dispersion and increases its stability,for example by forming around the individual grains layers that preventclumping. The dispersant may be for example a polymer. Generally, thedispersant breaks down at the latest during the sintering in step g).

A casting tool may be used for producing the abrasive grains accordingto the disclosure, the casting tool comprising at least one castingmold, which has at least one depression with a respective surface, thesurface being complementary to the form of at least part of the surfaceof the abrasive grain.

The casting mold may for example contain or consist of silicone. Thedepressions may have an open top surface, through which the dispersioncan be introduced. The depressions in the casting mold have in each casea surface of a form that is complementary to the form of at least partof the surface of the desired abrasive grain. Preferably, thedepressions in the casting mold have in each case a surface of a formthat is complementary to the form of the part of the surface of theabrasive grain that does not form the main face of the abrasive grain.

The precursor produced in step d) should preferably be mechanicallystable enough to be further processible as bulk material in thesubsequent steps. The optional calcining in step f) is advantageous, oreven required, in particular whenever the dispersion contains a numberof different raw materials and a phase transformation is required.

In a further development of the method known from U.S. Pat. No.5,201,916, for some abrasive grains the aforementioned casting mold mayadvantageously be only part of a multipart casting tool thatadditionally comprises at least one molding element, in particular afurther casting mold or die element, with which, in addition to thesurface molded in the first casting mold, at least part of the remainingsurface of the abrasive grain can be molded.

The die elements preferably do not close off the depressions in thecasting mold completely, and so the volatile component of the dispersioncan escape.

More complicated forms of bodies of abrasive grains can be formed incasting molds that can be assembled, in a way similar to in theinjection-molding process. For this purpose, at least one casting moldhas at least one filling opening, through which the dispersion can beintroduced into the depressions.

Another aspect of the disclosure relates to an abrasive article thatcontains a collective of abrasive grains as described above. Inparticular, it is thus possible for at least 20% by weight, preferablyat least 50% by weight, particularly preferably at least 90% by weight,of all the abrasive grains of the abrasive article to be formed asabrasive grains according to the disclosure, as described above. Theother abrasive grains may likewise have a defined form, but one whichdiffers from the form according to the disclosure, or they may not havea defined form.

The abrasive article may be for example a coated abrasive article(coated abrasive), a nonwoven abrasive article, a bonded abrasivearticle (bonded abrasive) or an abrasive brush.

A coated abrasive article includes a backing, in particular a flexiblebacking, such as for example paper, vulcanized fiber, a film, a textilematerial, a foam or multilayer combinations thereof. The abrasive grainscan be secured to the backing with the aid of a base binder (“makecoat”). The make coat and the abrasive grains may be covered with a topbinder (“size coat”). Optionally, it is also possible for there to beabove the size coat mentioned a second top binder (“supersize coat”).

All binders known per se, for example of synthetic resin, such as forinstance a phenolic resin, an epoxide, a urea resin, a melamine resin oran unsaturated polyester resin, may be used as the make coat, size coatand supersize coat. The size coat and/or supersize coat may also containfurther customary active ingredients and/or fillers.

The abrasive article may take different product forms, for example thatof an abrasive disk or that of an abrasive belt.

The disclosure also comprises a method for producing an abrasive articleaccording to the disclosure, as described above. The method includes astep in which a collective of abrasive grains is fixed on and/or in asubstrate, in particular by means of a binder. The substrate may be forinstance a backing, in particular a flexible backing, of a coatedabrasive article, a nonwoven material of a nonwoven abrasive, a matrixof a bonded abrasive or bristles of an abrasive brush. In the case of acoated abrasive article, the make coat and/or the abrasive grains and/orthe size coat and/or the supersize coat can be applied by a method knownper se. For example, the abrasive grains can be appliedelectrostatically or mechanically (i.e. gravimetrically). For manyembodiments according to the disclosure, even with mechanicalscattering, a large proportion of the abrasive grains are oriented on anabrasive material backing such that the main face comes to lie on thebacking. This applies in particular to abrasive grains in the form of acone or a frustocone, to be precise in particular whenever theaforementioned preferred ratios between the height and the area diameterof the abrasive grain are applicable. This makes it possible to dispensewith more complex electrostatic scattering.

Furthermore, the disclosure is also directed to a method for grinding asurface with an abrasive article as described above. The surface may bein particular a painted surface. In the case of a painted surface,abrasive grains with sizes of 500 μm or less are particularly suitable.

The object is also achieved by an abrasive grain with at least two mainsurfaces and at least one subsidiary surface, which is connected by wayof a first edge to a first main surface and is connected by way of asecond edge to a second main surface, which does not have any edge incommon with the first main surface. In this case, according to thedisclosure the subsidiary surface includes an obtuse angle with thefirst main surface in the region of the first edge and an obtuse anglewith the second main surface in the region of the second edge.

In particular if the subsidiary surface is planar, the two main surfacesare consequently not parallel to one another.

A main surface is understood as meaning a part of the area of thesurface of an abrasive grain that forms at least 5%, in particular atleast 10%, preferably at least 15% of the total surface of the abrasivegrain. In particular, the main surfaces together have a proportion of atleast 60%, preferably 70% of the total surface of the abrasive grain.

The main surfaces are preferably similar, more preferably congruent, toone another.

By analogy, a subsidiary surface is understood as meaning a part of thearea that is significantly smaller than the main surfaces and forms atmost 10%, preferably at most 5%, of the total surface of the abrasivegrain.

In a preferred embodiment, the ratio of the area of subsidiary surfaceto the area of the main surface is from 1 to 10, particularly preferablyfrom 2 to 15, most particularly preferably from 5 to 10.

Main and subsidiary surfaces are preferably substantially planar and aregenerally bounded by more than one edge.

The subsidiary surface preferably includes an obtuse angle with thefirst main surface and an obtuse angle with the second main surface ineach case along at least 80% of an edge, in particular along an entireedge.

The edges of the abrasive grain are preferably substantially straight.

The edges of the abrasive grain are preferably substantially of the samelength. Alternatively, the abrasive grain may have a compact orcompressed form, by the edges having different lengths. In particular,in a preferred embodiment, three first edges are substantially of thesame length and three second edges are likewise of the same length, butshorter than the three first edges. In this case, the three first edgeshave a first length L1 and the three second edges have a second lengthL2. The ratio of the second length L2 and the first length L1 ispreferably from 0.7 to 0.98, particularly preferably from 0.8 to 0.95,most particularly preferably from 0.85 to 0.90. This provides anadvantageous bimodal distribution of the abrasive grains on the abrasivearticle. Depending on how such an abrasive grain comes to lie on thebacking of the abrasive article, two different heights of the abrasivegrain on the backing are obtained.

An abrasive grain with such a form has a basic structure that is givenby the main surfaces and can be very simple. Nevertheless, the presenceof the subsidiary surfaces means that the body of the abrasive grain hasfurther edges, which increase the cutting force.

The object is also achieved by an abrasive grain with an abrasive grainbody of which the surface has at least two substantially planar mainsurfaces that lie on the faces of an imaginary convex polyhedron, inparticular a Platonic solid, an Archimedean solid, a Catalan solid, aprism or antiprism, on the faces of a linearly distorted Platonic solid,Archimedean solid, Catalan solid, prism or antiprism or on the faces ofan imaginary combination of the solids mentioned, the abrasive grainbody having at least one flatly truncated edge. The main surfaces are inthis case understood according to the above definition.

Platonic solids are those convex polyhedrons of which the side faces areall mutually congruent regular polygons, and of which an equal numbermeet at every corner. The Platonic solids include the tetrahedron,octahedron and icosahedron, each of which have equilateral triangles asside faces, the hexagon or cube, which have squares as side faces, andthe dodecahedron with equilateral pentagons as side faces.

The Archimedean solids are complex polyhedrons of which the side facesare regular polygons. The characteristic property of the Archimedeansolids is that all the corners of the solid are entirely identical toone another.

Prisms, antiprisms and the five Platonic solids are not counted amongthe Archimedean solids.

The Archimedean solids include in particular the Platonic solids withcapped corners (frustotetrahedron, frustohexahedron, frustooctahedron,frustododecahedron and frustoicosahedron).

A prism is bounded by precisely two congruent n-edged polygons and nparallelograms. The n-edged polygons may in particular be regularn-edged polygons. In the case of a straight prism, the parallelogramsmentioned are rectangles, in particular squares. Such a prism exists forevery natural number n greater than or equal to three. At a corner, onen-edged polygon and two parallelograms always meet.

An antiprism is bounded by precisely two congruent n-edged polygons and2n triangles, in particular equilateral triangles. Such an antiprismexists for every natural number n greater than or equal to three. At acorner, one n-edged polygon and three triangles always meet.

The Catalan solids have only one kind of face, that is identicalirregular polygons, but have at least two different kinds of corners.

In a linearly distorted Platonic solid, Archimedean solid, Catalansolid, prism or antiprism, i.e. one that is compressed or extended alongat least one axis, in particular an axis of symmetry, not all symmetriesare maintained compared to the undistorted form. Since, however, theforms mentioned have a multitude of symmetries, the linearly distortedcorresponding solid still has a number of symmetries. A cuboid forexample is a linearly distorted cube that has fewer symmetries than acube but still has many regularities.

In the case of an edge that is flatly truncated according to thedisclosure, instead of an edge of the convex polyhedron the facesadjoining this edge are bounded by a subsidiary surface. The surface ofthe abrasive grain therefore has at least one subsidiary surface thatlies on the area of the truncated edge.

A flatly truncated edge is obtained by cutting off from an imaginarybasic body a piece, including an entire edge, parallel to the edge. Thecutting surface area is preferably planar, whereby a planar subsidiarysurface is produced.

The form of the abrasive grain is therefore obtained by a comparativelysimple basic form that has further edges as a result of the truncationof at least one edge.

Also the subject of the present disclosure are such abrasive grainbodies that are made up of component bodies, one of the component bodiesbeing a convex polyhedron with at least one truncated edge. The othercomponent body may for example form a kind of base that consists of thesame or a different material. The abrasive grain body is preferablyformed as one piece. In particular, with mechanical scattering, theabrasive grain body preferably falls on the base, and the edges of theconvex polyhedron face away from the backing, for example on account ofgravitational force or some other external force.

In a preferred embodiment, an abrasive grain has a body as describedabove, and some of the faces, preferably at least three faces, of theabrasive grain, in particular main surfaces, lie on the faces of animaginary pyramid or an imaginary frustopyramid, in particular on thefaces of a tetrahedron.

Preferably, all of the edges of the tetrahedron are truncated, and sothe abrasive grain body has on the one hand a compact and stable formand at the same time a multiplicity of cutting edges. Moreover, theabrasive grain body is symmetrically formed, and so it does not matteron which main surface it comes to lie during the scattering.

Each main surface is advantageously connected to at least one other mainsurface by way of a subsidiary surface.

Since each surface is bounded by edges, consequently a multiplicity ofedges are obtained, which increases the cutting force of the abrasivegrain.

The form and the size of the abrasive grain can be determined forinstance with the aid of a microscope. The abrasive grain according tothe disclosure may have a size in the entire range of sizes that is alsocustomary for conventional abrasive grains. Usually, abrasive grainswith larger sizes lead to a greater removal of material from a workedsurface than smaller abrasive grains. For example, the abrasive grainmay have a size in the range from 100 μm to 2000 μm. This size can bedetermined experimentally with the aid of a microscope. It is understoodas the diameter of an enveloping circle of the microscoped image of theabrasive grain, that is to say the smallest diameter of a circle thatencloses the image.

The form described above of the abrasive grain is an idealization.However, the disclosure also comprises abrasive grains that deviate fromthis idealized form within the limits of production tolerances. Possibledeviations from the idealized form may be due to one or more of thefollowing causes:

cavities or small bubbles on account of trapped air and/or other gasesin a dispersion from which the abrasive grains are produced;

missing corners and/or edges that are produced due to incomplete fillingof a casting mold and/or during removal of a precursor of the abrasivegrain from a casting mold;

sunken side faces and/or edges that are produced due to shrinkage duringthe removal of part of the volatile components of the dispersion; inparticular, sunken faces that are produced from the upper free surfaceof the dispersion, which is not in contact with the casting mold;

instances of flaking that are caused by a drying and/or sinteringprocess;

broken-off corners and/or edges that are produced by transporting and/orduring further processing of the abrasive grains as bulk material.

The deviations from the idealization do not necessarily have to lead todisadvantageous properties of the abrasive grain. For example,broken-off corner and/or edges can also have the effect that furthercutting edges in comparison with the idealization are produced, and mayeven positively influence the abrasive effect.

The disclosure also comprises in particular abrasive grains of which theform only coincides substantially with the idealized form. For example,portions of the outer contour, in particular edges, are also regarded assubstantially straight if they are at least partially or even completelycurve-shaped and have a radius of curvature that is at least twice,preferably at least five times, particularly preferably at least tentimes, the size defined above of the abrasive grain. A point of theouter contour is regarded substantially as a corner if the radius ofcurvature there is at most 10%, preferably at most 5%, particularlypreferably at most 2%, of the size of the abrasive grain. Furthermore,main surfaces and subsidiary surfaces are understood as substantiallyplanar if they are curved and have radii of curvature that are at leasttwice, preferably at least five times, particularly preferably at leastten times, the size of the abrasive grain.

However, the abrasive grain preferably has an idealized formed asdescribed above.

The abrasive grain may for example contain or consist of a ceramicmaterial, in particular a polycrystalline ceramic material. The abrasivegrain preferably contains aluminum oxide, particularly preferablyα-Al₂O₃.

Alternatively or in addition, the abrasive grain may also contain atleast one further metal oxide, such as for instance sodium oxide,magnesium oxide, iron oxide, silicon oxide, calcium oxide, zirconiumoxide, yttrium oxide, zinc oxide, cobalt oxide, nickel oxide, hafniumoxide, chromium oxide, praseodymium oxide, samarium oxide, ytterbiumoxide, neodymium oxide, lanthanum oxide, gadolinium oxide, cerium oxide,dysprosium oxide, erbium oxide, lutetium oxide, titanium oxide,manganese oxide or any desired combinations thereof.

Many of these metal oxides originate from impurities in the starting rawmaterials, such as for example in aluminum oxide. With sufficientlysmall fractions in the abrasive grain, such impurities do not howeverhave any adverse influence on the production and application of theabrasive grain. Some of the impurities mentioned may even have apositive effect on the abrasive grain.

Fractions of zirconium oxide or yttrium oxide may for example originatefrom grinding balls that can be used in a grinding step in theproduction of the abrasive grains. Fractions of iron oxide may originatefrom a grinding vessel that is used in such a grinding step.

Likewise alternatively or in addition, the abrasive grain may containfurther hard materials, such as for example silicon carbide.

Furthermore, the abrasive grain may contain at least one breakdownproduct of a dispersant described in more detail below that was used inthe production of the abrasive grains. In addition, the abrasive grainmay comprise at least one nucleating agent or breakdown product thereofthat was used in the production of the abrasive grains. The nucleatingagent may be for example the magnesium oxide already mentioned above.

Moreover, the abrasive grain may also contain at least one of thefurther substances described in EP 615 816 A1.

The ingredients mentioned can be determined with the aid of chemicalanalysis methods known per se.

The abrasive grain may contain or consist of a structure having one ormore different phases. A first phase may consist of aluminum oxide,particularly preferably of α-Al₂O₃. A second phase may consist of one ormore of the aforementioned further metal oxides and/or further hardsubstances.

The proportion of aluminum oxide, in particular of α-Al₂O₃, in theabrasive grain may be for example at least 25% by weight, preferably atleast 50% by weight, more preferably at least 70% by weight,particularly preferably at least 95% by weight.

The abrasive grain may have a coating which covers only part of thesurface, in particular only one or more edges and/or only one of anumber of flat regions of the surface. The coating may for instance be aferromagnetic or paramagnetic coating. Such a partial coating of thesurface with a ferromagnetic or paramagnetic material makes it possibleto align the abrasive grain in a given direction in a magnetic fieldapplied during the scattering operation. Alternatively, it may also be acoating of a material with increased thermal conductivity or a coatingthat makes enhanced adhesion of the abrasive grain on the abrasivematerial backing possible.

A further aspect of the disclosure relates to a collective of abrasivegrains. A collective of abrasive grains is understood here andhereinafter as meaning a coherent collection of abrasive grains. Forexample, this may be a collection of abrasive grains that are containedin a container and are stored and/or transported as such, for example ina sack.

Such a collection of abrasive grains can be used to produce an abrasivearticle. The entirety of all the abrasive grains present in an abrasivearticle is also regarded as a collective of abrasive grains.

Preferably, the collective of abrasive grains includes at least 20% byweight, preferably at least 50% by weight, particularly preferably atleast 90% by weight, of abrasive grains according to the disclosure, asdescribed above. The other abrasive grains included in the collectivemay likewise have a defined form, but one that differs from the formaccording to the disclosure, or they may not have a defined form sincethey are for example fractured abrasive grains. These other abrasivegrains included in the collective are also referred to as “supportinggrains”.

It is conceivable and within the scope of the disclosure that theabrasive grains according to the disclosure included in the collectiveare formed differently from one another. For example, the collective ofabrasive grains may include a first fraction of abrasive grains of afirst embodiment according to the disclosure and also a second fractionof abrasive grains of a second embodiment according to the disclosurethat is different from the first embodiment according to the disclosure.In particular, the abrasive grains of the first embodiment according tothe disclosure may differ from the abrasive grains of the secondembodiment according to the disclosure in their size and/or in theirform.

The collective of abrasive grains may consist exclusively of identicalabrasive grains according to the disclosure; in particular, thecollective then has a size distribution in the form of points.

The collective of abrasive grains may substantially have a sizedistribution that conforms to a size standard customary in the abrasivesindustry, for example the American National Standards Institute (ANSI),the Standards of the Federation of European Producers of Abrasives(FEPA) or the Japanese Industrial Standard (JIS). For example, thecollective of the abrasive grains may substantially have a grain size ofP12, P16, P20, P24, P30, P36, P40, P50, P60, P80, P100, P120, P150,P180, P220, P240, P280, P320, P360, P400, P500, P600, P800, P1000,P1200, P1500, P2000, P2500, P3000 or P5000 according to the FEPAstandard. In this context, “substantially” have a size distributionmeans that at least 90% by weight, preferably at least 95% by weight,more preferably at least 99% by weight and particularly preferably allof the abrasive grains in the collective of abrasive grains meet thisstandard.

As already described above, it is also conceivable that the collectiveincludes at least two different fractions of abrasive grains accordingto the disclosure and/or at least one fraction of abrasive grains notaccording to the disclosure. Each of these fractions may itself have asize distribution that respectively conforms to one of theaforementioned size standards customary in the abrasives industry.

An abrasive grain according to the disclosure or a collective ofabrasive grains according to the disclosure can for example be producedby the following method known from U.S. Pat. No. 5,201,916:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain.

Before and/or during the preparation of the dispersion in step a), theraw materials, in particular α-alumina particles and/or particles thatcan be converted into α-alumina, may be ground. This may be performedfor example with the aid of a ball mill, in particular with the aid of aplanetary ball mill.

The dispersion may contain at least one dispersant. Such a dispersantfacilitates the formation of the dispersion and increases its stability,for example by forming around the individual grains layers that preventclumping. The dispersant may be for example a polymer. Generally, thedispersant breaks down at the latest during the sintering in step g).

A casting tool may be used for producing the abrasive grains accordingto the disclosure, the casting tool comprising at least one castingmold, which has at least one depression with a respective surface, thesurface being complementary to the form of at least part of the surfaceof the abrasive grain.

There are preferably a multiplicity of depressions in the casting mold,and so a multiplicity of bodies of abrasive grains can be cast by onecasting operation.

In a further development of the method known from U.S. Pat. No.5,201,916, for the production of some embodiments according to thedisclosure the aforementioned casting mold may advantageously be onlypart of a multipart casting tool that additionally comprises at leastone molding element, in particular a further casting mold or dieelement, with which, in addition to the surface molded in the firstcasting mold, at least part of the remaining surface of the abrasivegrain can be molded. For example, die elements that are guided into thefilled depressions in the first casting mold may be provided.

The die elements preferably do not close off the depressions in thecasting mold completely, and so the volatile component of the dispersioncan escape.

More complicated forms of bodies of abrasive grains can be formed incasting molds that can be assembled, in a way similar to in theinjection-molding process. For this purpose, at least one casting moldhas at least one filling opening, through which the dispersion can beintroduced into the depressions.

The casting mold and/or the die element may for example contain orconsist of silicone. The depressions may have an open top surface,through which the dispersion can be introduced. The depressions in thecasting mold have in each case a surface of a form that is complementaryto the form of at least part of the surface of the desired abrasivegrain.

The precursor produced in step d) should preferably be mechanicallystable enough to be further processible as bulk material in thesubsequent steps. The optional calcining in step f) is advantageous, oreven required, in particular whenever the dispersion contains a numberof different raw materials and a phase transformation is required.

Another aspect of the disclosure relates to an abrasive article thatcontains a collective of abrasive grains as described above. Inparticular, it is thus possible for at least 20% by weight, preferablyat least 50% by weight, particularly preferably at least 90% by weight,of all the abrasive grains of the abrasive article to be formed asabrasive grains according to the disclosure, as described above. Theother abrasive grains may likewise have a defined form, but one whichdiffers from the form according to the disclosure, or they may not havea defined form.

The abrasive article may be for example a coated abrasive article(coated abrasive), a nonwoven abrasive article, a bonded abrasivearticle (bonded abrasive) or an abrasive brush.

A coated abrasive article includes a backing, in particular a flexiblebacking, such as for example paper, vulcanized fiber, a film, a textilematerial, a foam or multilayer combinations thereof. The abrasive grainscan be secured to the backing with the aid of a base binder (“makecoat”). The make coat and the abrasive grains may be covered with a topbinder (“size coat”). Optionally, it is also possible for there to beabove the size coat mentioned a second top binder (“supersize coat”).

All binders known per se, for example of synthetic resin, such as forinstance a phenolic resin, an epoxide, a urea resin, a melamine resin oran unsaturated polyester resin, may be used as the make coat, size coatand supersize coat. The size coat and/or supersize coat may also containfurther customary active ingredients and/or fillers.

The abrasive article may take different product forms, for example thatof an abrasive disk or that of an abrasive belt.

The disclosure also comprises a method for producing an abrasive articleaccording to the disclosure, as described above. The method includes astep in which a collective of abrasive grains is fixed on and/or in asubstrate, in particular by means of a binder. The substrate may be forinstance a backing, in particular a flexible backing, of a coatedabrasive article, a nonwoven material of a nonwoven abrasive, a matrixof a bonded abrasive or bristles of an abrasive brush. In the case of acoated abrasive article, the make coat and/or the abrasive grains and/orthe size coat and/or the supersize coat can be applied by a method knownper se. For example, the abrasive grains can be appliedelectrostatically or mechanically (i.e. gravimetrically). On account ofthe form according to the disclosure of the abrasive grains, even withmechanical scattering, a large proportion of the abrasive grains areoriented on an abrasive material backing such that a multiplicity ofedges face away from the backing. This makes it possible to dispensewith more complex electrostatic scattering.

Furthermore, the disclosure is also directed to a method for grinding asurface with an abrasive article as described above. The surface may bein particular a painted surface. In the case of a painted surface,abrasive grains with sizes of 500 μm or less are particularly suitable.

The object is also achieved by an abrasive grain with a defined form,the abrasive grain having a body which is simply connected and has asurface with at least two faces, in particular at least five faces,preferably at least seven faces, one face being formed as a base area.According to the disclosure, the contour of the base area has at leastone reflex angle.

In the present application, a simply connected body is understood asmeaning a body in which every continuous path on the body can be reducedto a point. The body accordingly does not have any holes passing rightthrough it.

A reflex angle is an angle that is greater than 180°.

Usually, the angle in the base area defines a corner at which edgesmeet. In the case of a reflex angle in the contour of the base area,there is generally an edge at which two faces include a reflex angle.While a convex body is understood as meaning a body without“indentations” and without holes, according to the disclosure theabrasive grain body is not convex, but has an “indentation”.

The “indentation” has the consequence that elsewhere, for example at theperiphery of the “indentation”, a corner and/or an edge with an acuteangle must occur between mutually converging faces and/or elsewherethere must be a convex curvature with smaller radii of curvature thanwould be the case without the “indentation”.

These indirectly defined pointed corners, sharp edge or greatly curvedfaces contribute to the improved cutting force of an abrasive grain.

In an advantageous solution of the disclosure, the abrasive grain hasthe form of a cone or a frustocone or has at least a component body ofthis form. That the abrasive grain includes a component body should notbe understood in the sense that the abrasive grain has been or isnecessarily joined together from two or more initially separatelyproduced component bodies. Instead, the abrasive grains according to thedisclosure are preferably formed as one piece.

A cone is understood here generally as meaning a geometrical body thatis bounded by a substantially planar base area and substantiallystraight generating lines, a generating line respectively extending fromeach point of the outer contour of the base area to a common point ofthe cone (the vertex of the cone) (and so all of the generating lineslie between the contour of the base area and the vertex of the cone),the vertex of the cone lying outside the plane defined by the base area.This definition is therefore not restricted to circular cones nor tocones with a polygonal base area nor to straight cones. The base area ofthe abrasive grain, in particular the base area of the cone, has atleast one reflex angle.

In particular, the cone may be a pyramid in which the base area is apolygon. A pyramid is a polyhedron that consists of a number ofsubstantially planar faces lying one against the other, of which one isa polygon, in the present case a concave polygon, and all the others aretriangles. The triangles (side faces) form the lateral area.

For the purposes of the present disclosure, the form of a frustocone isproduced by cutting off from the form of a cone a smaller cone. Inparticular, a smaller cone that is similar to the original cone may becut off from a cone parallel to the base area. This definition is alsoequally not restricted to polygonal base areas. If the base area is apolygon, the frustocone is a frustopyramid. The scope of the presentdisclosure is intended also to include those bodies that are produced bya cut that is not parallel to the base area. The frustopyramidaccordingly consists of a base area, a number of quadrangles lying oneagainst the other and a likewise polygonal top area.

The scope of the present disclosure also includes abrasive grains with abody that is bounded by a number of substantially planar faces lying oneagainst the other, of which one, a concave polygon, forms a base area,the side faces are triangles or quadrangles that form base edges withthe base area and form side edges with one another, the side edgesconverging with one another but not necessarily having to be in linewith a point as in the case of a conventional cone or a conventionalfrustocone, in particular a conventional pyramid or a conventionalfrustopyramid.

The bodies claimed here with a polygonal base area and with side facestapering with increasing distance from the base area taper overall withincreasing distance from the base area. They therefore have a center ofgravity in the vicinity of the base area and, with mechanical scatteringonto a backing, preferably fall on the base area.

The edges and corners facing away from the backing, and consequently theabrasive effect, can be defined by the choice of the specific body form.In the present case, the concave contour of the base area ensures thatenough sharp edges face away from the backing.

The concave base area is formed in particular by a star-shaped base areaof the cone or frustocone, in particular of the pyramid or thefrustopyramid.

The sharp edges that extend from the points of the star point indifferent spatial directions. When there are a multiplicity of abrasivegrains applied randomly to a backing, there are consequently amultiplicity of sharp abrasive edges distributed uniformly in alldirections. The abrasive material can consequently be used well for allgrinding directions, without any particular preferential direction.

In an advantageous embodiment of the disclosure, the abrasive grain or acomponent body of the abrasive grain has the form of a cylinder, inparticular a prism, or an antiprism.

A cylinder is understood here and hereinafter as meaning a solid that isbounded by two base areas and also a lateral area. The base areas are inthis case preferably substantially planar and likewise preferablysubstantially parallel to one another. The lateral area is formed by anarray of lines that are substantially parallel to one another.Therefore, in particular, the form of the base areas is not restrictedto polygons or circles.

If the base area is a polygon, the abrasive grain or its component bodyhas the form of a prism. A prism is bounded by precisely two congruentn-edged polygons and n parallelograms. The n-edged polygons may inparticular be regular n-edged polygons. In the case of a straight prism,the parallelograms mentioned are rectangles, in particular squares. Sucha prism exists for every natural number n greater than or equal tothree. At a corner, one n-edged polygon and two parallelograms alwaysmeet.

An antiprism is bounded by precisely two congruent n-edged polygons and2n triangles, in particular equilateral triangles. Such an antiprismexists for every natural number n greater than or equal to three. At acorner, one n-edged polygon and three triangles always meet.

The linearly distorted prisms and antiprisms are also the subject of thepresent application.

In a linearly distorted prism or antiprism, i.e. one that is compressedor extended along at least one axis, in particular an axis of symmetry,not all symmetries are maintained compared to the undistorted form.Since, however, the forms mentioned have a multitude of symmetries, thelinearly distorted corresponding solid still has a number of symmetries.

According to the disclosure, the congruent n-edged polygons of which oneforms the base area have a concave contour. In particular, the base areais formed by a star-shaped polygon.

Simple geometrical solids, such as prisms or antiprisms, with a concavepolygon contour, can be reproducibly produced in a comparativelyundemanding manner.

Solids of this kind usually have a multitude of symmetries, and so forthe individual bodies a number of equivalent alignments are obtained ona backing. When scattering abrasive grains onto a backing, asufficiently large number of pointed edges and/or corners and/or moregreatly curved surfaces face in the grinding direction.

In an advantageous embodiment of the abrasive grain according to thedisclosure, the contour of the base area comprises at least one segmentof a circle, preferably at least two and particularly preferablyprecisely two segments of a circle, which are preferably arrangedsymmetrically in relation to one another. In particular, the base areacorresponds to the sectional image of a double-T anchor. The abrasivegrain preferably has a body that is theoretically made up of a prism ofwhich the base area corresponds to the sectional image of a double-Tanchor and a saddleback roof-shaped cap.

The form and the size of the abrasive grain can be determined forinstance with the aid of a microscope. The abrasive grain according tothe disclosure may have a size in the entire range of sizes that is alsocustomary for conventional abrasive grains. Usually, abrasive grainswith larger sizes lead to a greater removal of material from a workedsurface than smaller abrasive grains. For example, the abrasive grainmay have a size in the range from 100 μm to 2000 μm. This size can bedetermined experimentally with the aid of a microscope. It is understoodas the diameter of an enveloping circle of the microscoped image of theabrasive grain, that is to say the smallest diameter of a circle thatencloses the image.

The form described above of the abrasive grain is an idealization.However, the disclosure also comprises abrasive grains that deviate fromthis idealized form within the limits of production tolerances. Possibledeviations from the idealized form may be due to one or more of thefollowing causes:

cavities or small bubbles on account of trapped air and/or other gasesin a dispersion from which the abrasive grains are produced;

missing corners and/or edges that are produced due to incomplete fillingof a casting mold and/or during removal of a precursor of the abrasivegrain from a casting mold;

sunken side faces and/or edges that are produced due to shrinkage duringthe removal of part of the volatile components of the dispersion; inparticular, sunken faces that are produced from the upper free surfaceof the dispersion, which is not in contact with the casting mold;

instances of flaking that are caused by a drying and/or sinteringprocess;

broken-off corners and/or edges that are produced by transporting and/orduring further processing of the abrasive grains as bulk material.

The deviations from the idealization do not necessarily have to lead todisadvantageous properties of the abrasive grain. For example,broken-off corner and/or edges can also have the effect that furthercutting edges in comparison with the idealization are produced, and mayeven positively influence the abrasive effect.

The disclosure also comprises in particular abrasive grains of which theform only coincides substantially with the idealized form. However, theabrasive grain preferably has an idealized formed as described above.

The abrasive grain may for example contain or consist of a ceramicmaterial, in particular a polycrystalline ceramic material. The abrasivegrain preferably contains aluminum oxide, particularly preferablyα-Al₂O₃.

Alternatively or in addition, the abrasive grain may also contain atleast one further metal oxide, such as for instance sodium oxide,magnesium oxide, iron oxide, silicon oxide, calcium oxide, zirconiumoxide, yttrium oxide, zinc oxide, cobalt oxide, nickel oxide, hafniumoxide, chromium oxide, praseodymium oxide, samarium oxide, ytterbiumoxide, neodymium oxide, lanthanum oxide, gadolinium oxide, cerium oxide,dysprosium oxide, erbium oxide, lutetium oxide, titanium oxide,manganese oxide or any desired combinations thereof.

Many of these metal oxides originate from impurities in the starting rawmaterials, such as for example in aluminum oxide. With sufficientlysmall fractions in the abrasive grain, such impurities do not howeverhave any adverse influence on the production and application of theabrasive grain. Some of the impurities mentioned may even have apositive effect on the abrasive grain.

Fractions of zirconium oxide or yttrium oxide may for example originatefrom grinding balls that can be used in a grinding step in theproduction of the abrasive grains. Fractions of iron oxide may originatefrom a grinding vessel that is used in such a grinding step.

Likewise alternatively or in addition, the abrasive grain may containfurther hard materials, such as for example silicon carbide.

Furthermore, the abrasive grain may contain at least one breakdownproduct of a dispersant described in more detail below that was used inthe production of the abrasive grains. In addition, the abrasive grainmay comprise at least one nucleating agent or breakdown product thereofthat was used in the production of the abrasive grains. The nucleatingagent may be for example the magnesium oxide already mentioned above.

Moreover, the abrasive grain may also contain at least one of thefurther substances described in EP 615 816 A1.

The ingredients mentioned can be determined with the aid of chemicalanalysis methods known per se.

The abrasive grain may contain or consist of a structure having one ormore different phases. A first phase may consist of aluminum oxide,particularly preferably of α-Al₂O₃. A second phase may consist of one ormore of the aforementioned further metal oxides and/or further hardsubstances.

The proportion of aluminum oxide, in particular of α-Al₂O₃, in theabrasive grain may be for example at least 25% by weight, preferably atleast 50% by weight, more preferably at least 70% by weight,particularly preferably at least 95% by weight.

The abrasive grain may have a coating which covers only part of thesurface, in particular only one or more edges and/or only one of anumber of flat regions of the surface. The coating may for instance be aferromagnetic or paramagnetic coating. Such a partial coating of thesurface with a ferromagnetic or paramagnetic material makes it possibleto align the abrasive grain in a given direction in a magnetic fieldapplied during the scattering operation. Alternatively, it may also be acoating of a material with increased thermal conductivity or a coatingthat makes enhanced adhesion of the abrasive grain on the abrasivematerial backing possible. In particular, the coating may only bepresent on the base area of the abrasive grain.

A further aspect of the disclosure relates to a collective of abrasivegrains. A collective of abrasive grains is understood here andhereinafter as meaning a coherent collection of abrasive grains. Forexample, this may be a collection of abrasive grains that are containedin a container and are stored and/or transported as such, for example ina sack.

Such a collection of abrasive grains can be used to produce an abrasivearticle. The entirety of all the abrasive grains present in an abrasivearticle is also regarded as a collective of abrasive grains.

Preferably, the collective of abrasive grains includes at least 20% byweight, preferably at least 50% by weight, particularly preferably atleast 90% by weight, of abrasive grains according to the disclosure, asdescribed above. The other abrasive grains included in the collectivemay likewise have a defined form, but one that differs from the formaccording to the disclosure, or they may not have a defined form sincethey are for example fractured abrasive grains. These other abrasivegrains included in the collective are also referred to as “supportinggrains”.

It is conceivable and within the scope of the disclosure that theabrasive grains according to the disclosure included in the collectiveare formed differently from one another. For example, the collective ofabrasive grains may include a first fraction of abrasive grains of afirst embodiment according to the disclosure and also a second fractionof abrasive grains of a second embodiment according to the disclosurethat is different from the first embodiment according to the disclosure.In particular, the abrasive grains of the first embodiment according tothe disclosure may differ from the abrasive grains of the secondembodiment according to the disclosure in their size and/or in theirform.

The collective of abrasive grains may consist exclusively of identicalabrasive grains according to the disclosure; in particular, thecollective then has a size distribution in the form of points.

The collective of abrasive grains may substantially have a sizedistribution that conforms to a size standard customary in the abrasivesindustry, for example the American National Standards Institute (ANSI),the Standards of the Federation of European Producers of Abrasives(FEPA) or the Japanese Industrial Standard (JIS). For example, thecollective of the abrasive grains may substantially have a grain size ofP12, P16, P20, P24, P30, P36, P40, P50, P60, P80, P100, P120, P150,P180, P220, P240, P280, P320, P360, P400, P500, P600, P800, P1000,P1200, P1500, P2000, P2500, P3000 or P5000 according to the FEPAstandard. In this context, “substantially” have a size distributionmeans that at least 90% by weight, preferably at least 95% by weight,more preferably at least 99% by weight and particularly preferably allof the abrasive grains in the collective of abrasive grains meet thisstandard.

As already described above, it is also conceivable that the collectiveincludes at least two different fractions of abrasive grains accordingto the disclosure and/or at least one fraction of abrasive grains notaccording to the disclosure. Each of these fractions may itself have asize distribution that respectively conforms to one of theaforementioned size standards customary in the abrasives industry.

An abrasive grain according to the disclosure or a collective ofabrasive grains according to the disclosure can for example be producedby the following method known from U.S. Pat. No. 5,201,916:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain.

Before and/or during the preparation of the dispersion in step a), theraw materials, in particular α-alumina particles and/or particles thatcan be converted into α-alumina, may be ground. This may be performedfor example with the aid of a ball mill, in particular with the aid of aplanetary ball mill.

The dispersion may contain at least one dispersant. Such a dispersantfacilitates the formation of the dispersion and increases its stability,for example by forming around the individual grains layers that preventclumping. The dispersant may be for example a polymer. Generally, thedispersant breaks down at the latest during the sintering in step g).

A casting tool may be used for producing the abrasive grains accordingto the disclosure, the casting tool comprising at least one castingmold, which has at least one depression with a respective surface, thesurface being complementary to the form of at least part of the surfaceof the abrasive grain.

The casting mold may for example contain or consist of silicone. Thedepressions may have an open top surface, through which the dispersioncan be introduced. The depressions in the casting mold have in each casea surface of a form that is complementary to the form of at least partof the surface of the desired abrasive grain.

Preferably, the depressions are formed such that they have areas andedges that are complementary to the areas and edges facing away from thebase area of the abrasive grain. When the dispersion is poured into thedepressions in the casting mold, the base area can then be produced onthe free surface of the casting mold and does not have to be moldedseparately.

Alternatively, the recesses may be formed such that at least one area iscomplementary to the base area of the abrasive grain body.

A die element or a further casting mold may then be necessary to mold ina defined manner the areas and edges of the abrasive grain body that arenot molded in the recesses in the first casting mold. The abrasive grainmay for example be molded in a two-part casting tool, in a way similarto in an injection-molding process. In this case, if necessary, apressure can be exerted on the dispersion to be molded, in order forexample to achieve filled molds and planar faces.

The precursor produced in step d) should preferably be mechanicallystable enough to be further processible as bulk material in thesubsequent steps. The optional calcining in step f) is advantageous, oreven required, in particular whenever the dispersion contains a numberof different raw materials and a phase transformation is required.

Another aspect of the disclosure relates to an abrasive article thatcontains a collective of abrasive grains as described above. Inparticular, it is thus possible for at least 20% by weight, preferablyat least 50% by weight, particularly preferably at least 90% by weight,of all the abrasive grains of the abrasive article to be formed asabrasive grains according to the disclosure, as described above. Theother abrasive grains may likewise have a defined form, but one whichdiffers from the form according to the disclosure, or they may not havea defined form.

The abrasive article may be for example a coated abrasive article(coated abrasive), a nonwoven abrasive article, a bonded abrasivearticle (bonded abrasive) or an abrasive brush.

A coated abrasive article includes a backing, in particular a flexiblebacking, such as for example paper, vulcanized fiber, a film, a textilematerial, a foam or multilayer combinations thereof. The abrasive grainscan be secured to the backing with the aid of a base binder (“makecoat”). The make coat and the abrasive grains may be covered with a topbinder (“size coat”). Optionally, it is also possible for there to beabove the size coat mentioned a second top binder (“supersize coat”).

All binders known per se, for example of synthetic resin, such as forinstance a phenolic resin, an epoxide, a urea resin, a melamine resin oran unsaturated polyester resin, may be used as the make coat, size coatand supersize coat. The size coat and/or supersize coat may also containfurther customary active ingredients and/or fillers.

The abrasive article may take different product forms, for example thatof an abrasive disk or that of an abrasive belt.

The disclosure also comprises a method for producing an abrasive articleaccording to the disclosure, as described above. The method includes astep in which a collective of abrasive grains is fixed on and/or in asubstrate, in particular by means of a binder. The substrate may be forinstance a backing, in particular a flexible backing, of a coatedabrasive article, a nonwoven material of a nonwoven abrasive, a matrixof a bonded abrasive or bristles of an abrasive brush. In the case of acoated abrasive article, the make coat and/or the abrasive grains and/orthe size coat and/or the supersize coat can be applied by a method knownper se. For example, the abrasive grains can be appliedelectrostatically or mechanically (i.e. gravimetrically). On account ofthe form according to the disclosure of the abrasive grains, even withmechanical scattering, a large proportion of the abrasive grains areoriented such that there is the advantage described above. This makes itpossible to dispense with more complex electrostatic scattering.

Furthermore, the disclosure is also directed to methods for grinding asurface with an abrasive article as described above. The surface may bein particular a painted surface. In the case of a painted surface,abrasive grains with sizes of 500 μm or less are particularly suitable.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is explained in more detail below with the aid of anumber of exemplary embodiments and drawings, in which:

FIGS. A1 a and b show a first embodiment of an abrasive grain accordingto the disclosure in two views;

FIGS. A2 a and b show a second embodiment of an abrasive grain accordingto the disclosure in two views;

FIG. A3 shows a third embodiment of an abrasive grain according to thedisclosure; and

FIG. A4 shows a fourth embodiment of an abrasive grain according to thedisclosure.

FIGS. B1 a and b show two views of a further embodiment of an abrasivegrain according to the disclosure in the form of a pyramid with aseven-cornered base area; and

FIGS. B2 a and b show two views of a further embodiment of an abrasivegrain according to the disclosure in the form of a frustopyramid with anon-convex base area.

FIG. C1 a shows a further embodiment of an abrasive grain according tothe disclosure in a perspective view;

FIG. C1 b shows the embodiment of an abrasive grain according to thedisclosure in a first side view;

FIG. C1 c shows the embodiment of an abrasive grain according to thedisclosure in a second side view; and

FIG. C1 d shows the embodiment of an abrasive grain according to thedisclosure in a plan view.

FIG. D1 a shows a further embodiment of an abrasive grain according tothe disclosure in a perspective view;

FIG. D1 b shows the embodiment of an abrasive grain according to thedisclosure according to FIG. D1 a in a plan view;

FIG. D2 a shows a further embodiment of an abrasive grain according tothe disclosure in a perspective view;

FIG. D2 b shows the embodiment of an abrasive grain according to thedisclosure according to FIG. D2 a in a plan view of the base area;

FIG. D3 a shows a further embodiment of an abrasive grain according tothe disclosure in a perspective view;

FIG. D3 b shows the embodiment of an abrasive grain according to thedisclosure according to FIG. D3 a in a plan view; and

FIG. D3 c shows the embodiment of an abrasive grain according to thedisclosure according to FIG. D3 a in a side view.

DETAILED DESCRIPTION

The abrasive grain 110 represented in a perspective view in FIG. A1 aincludes a planar, circular first face 120 with a first outer contour121 and a square, that is to say polygonal, second face 125 with asecond outer contour 126, parallel to the first face. Between the firstface 120 and the second face 125 there is formed a curved lateral face130. The lines drawn on the lateral face 130 indicate the curved shapeof the lateral area 130; however, they do not represent actual edges ofthe abrasive grain 110. The abrasive grain 110 extends completelybetween the two planes that are defined by the first face 120 and thesecond face 125.

The first outer contour 121 has no corner, since the direction of thetangent to the points of the first outer contour 121 runs continuously.The absence of corners in the first outer contour 121 contributes to thesecure anchorage of the abrasive grain 110 in a make coat. By contrastwith the first outer contour 121, the second outer contour 126 has fourcorners 127. Both these corners 127 and the four edges 128 extendingbetween them provide an abrasive effect.

The perpendicular projection of the second face 125 onto the first face120 extends completely within the first face 120, as can also be seenwell in FIG. A1 b. As a result, the abrasive grain 110 is particularlystable with respect to the tilting forces occurring during grinding ifit lies with the first face 120 on an abrasive material backing that isnot represented here.

FIG. A1 b shows a lateral sectional view through the sectional plane Srepresented in FIG. A1 a. This sectional plane S extends perpendicularlyto the first face 120 and the second face 125 and through the centerpoints of these two faces 120, 125. The abrasive grain 110 tapers alongthe direction R, which runs from the first face 120 perpendicularly tothe second face 125. As a result, particularly good anchorage in a makecoat applied to a backing can be achieved. Moreover, even with the aidof mechanical scattering, the abrasive grain 110 can with greatprobability be placed with the first face 120 on the backing. In thisorientation, the corners 127 and the edges 128 also face away from thebacking, and can consequently cause an abrasive effect. The sectionalline of the lateral area 130 with this sectional plane S extends in astraight line.

A further embodiment according to the disclosure is represented in FIGS.A2 a and A2 b. This abrasive grain 210 also has a planar circular firstface 220 with a first outer contour 221 and a square second face 225with a second outer contour 226, parallel to the first face. By contrastwith the abrasive grain 110 according to FIGS. A1 a and A1 b, thesectional line of the lateral area 230 of the abrasive grain 210 with asectional plane S is convex, as can be seen in FIG. A2 b. This convexityhas the effect of producing in the region of the first face 220 a kindof “standing foot”, with which the abrasive grain 210 can be anchoredparticularly stably in a make coat.

The abrasive grain 310 shown in FIG. A3 has a planar first face 320 witha first outer contour 321, which though not circular, similarly has nocorners. The opposite second face 325, extending parallel to the firstface, has the form of an irregular pentagon with five corners 327 andfive edges 328 extending in between. This abrasive grain 310 does nottaper in the direction R running perpendicularly from the first face320.

Finally, the abrasive grain 410 according to FIG. A4 also has a planarfirst face 420 with a first outer contour 421 and a second face 425 witha second outer contour 426, extending parallel to the first face. Thesecond outer contour 426 has four edges 428, 428′, 428″. Of these edges,the edge 428 is straight; the edges 428′ are convexly curved; the edge428″ is concavely curved. At the point where two edges 428, 428′, 428″touch, a corner 427 is respectively formed; this is so because at thesepoints the direction of the tangent to the second outer contour 426 runsdiscontinuously.

The abrasive grains according to the disclosure may be produced forexample by a method described hereinafter: firstly, a dispersion of 200g of α-Al₂O₃, 0.4 g of MgO, 90 g of water as a dispersion medium and 0.5g of dispersant is prepared. The MgO functions here as a nucleatingagent. The product Dolapix CE64, obtainable from Zschimmer & Schwarz,56108 Lahnstein, Germany, may be used for example as the dispersant. Thedispersion thus obtained is ground for 30 minutes at 200 revolutions perminute in a planetary ball mill, for example a planetary ball millPM400, obtainable from Retsch GmbH, 42781 Haan, Germany. Subsequently,the ground dispersion is introduced into a silicone casting moldcontaining depressions in the form of the desired abrasive grains. Afterthat, the volatile component, that is the water, is removed from thedispersion. This produces an abrasive grain precursor, which is removedfrom the casting mold. The first face of the abrasive grain can then beproduced from the upper free surface of the dispersion, which is not incontact with the casting mold. In a final step, the precursor issintered as bulk material at 1550° C. for 5 minutes. The dispersant isburnt out in the course of sintering.

An abrasive article according to the disclosure may for example beproduced as follows: on a backing of vulcanized fiber with a thicknessof 0.8 mm, a phenolic resin dispersion is applied in an amount of 120g/m² as a make coat precursor. Subsequently, 600 g/m² of the abrasivegrains according to the disclosure are applied by means of electrostaticscattering. After that, the make coat precursor is cured to give a makecoat. On top of the make coat and the abrasive grains, a phenolic resindispersion is applied in an amount of 800 g/m² as a size coat precursor,which is likewise cured.

The abrasive grain 110 shown in FIGS. B1 a and B1 b is formed as apyramid with a planar base area 111 in the form of a regular heptagon.It is therefore a special cone, in the sense of the definition usedhere, with a convex polygonal base area. The outer contour 112 of thebase area 113 includes seven corners 113 and seven edges 114, whichrespectively connect two of the corners 113 to one another. Each of thecorners 113 of the outer contour 112 of the base area 111 is connectedby way of a respective edge 116 to a vertex 115 of the pyramid, whichlikewise forms a corner of the abrasive grain 110. Altogether, theabrasive grain 110 therefore includes eight corners 113, 115 andfourteen edges 114, 116.

The ratio of the height h and the area diameter (not represented here)of the base area 111 may lie in the range from 0.8 to 1.4. As explainedabove, it is advantageous for mechanical scattering if the ratiomentioned tends to be small, since, with mechanical scattering, theabrasive grain is then preferably oriented with its base area on anabrasive material backing such that the vertex 115 faces away from thisbacking. In terms of advantageous chip-forming behavior, on the otherhand, greater ratios tend to be expedient.

The abrasive grain 210 according to FIGS. B2 a and B2 b is formed as afrustopyramid with a base area 211. According to FIG. B2 b, the basearea 211 has altogether 20 corners 213, 213′, which are connected to oneanother by 20 edges 214. Of the 20 corners, the twelve corners 213 havean internal angle of 90°, while the eight corners 213′ have an internalangle of 270°. The base area 211 is therefore not a convex polygon. Thetop area 217 of the pyramid is similar to the base area 211; the twoareas can therefore be transformed one into the other by a combinationof displacement and homothety. The top area 217 consequently alsoincludes 20 corners 218, 218′ and 20 edges 219. Between the base area211 and the top area 217 there extend 20 further edges 216, whichrespectively connect a corner 213, 213′ of the base area 211 to acorresponding corner 218, 218′ of the top area 217. The high numbers ofedges 216, 219 and of corners 218, 218′ provide a high cutting effect.

The abrasive grains according to the disclosure may be produced forexample by a method described hereinafter: firstly, a dispersion of 200g of α-Al₂O₃, 0.4 g of MgO, 90 g of water as a dispersion medium and 0.5g of dispersant is prepared. The MgO functions here as a nucleatingagent. The product Dolapix CE64, obtainable from Zschimmer & Schwarz,56108 Lahnstein, Germany, may be used for example as the dispersant. Thedispersion thus obtained is ground for 30 minutes at 200 revolutions perminute in a planetary ball mill, for example a planetary ball millPM400, obtainable from Retsch GmbH, 42781 Haan, Germany. Subsequently,the ground dispersion is introduced into a silicone casting moldcontaining depressions in the form of the desired abrasive grains. Afterthat, the volatile component, that is the water, is removed from thedispersion. This produces an abrasive grain precursor, which is removedfrom the casting mold. In a final step, the precursor is sintered asbulk material at 1550° C. for 5 minutes. The dispersant is burnt out inthe course of sintering.

An abrasive article according to the disclosure may for example beproduced as follows: on a backing of vulcanized fiber with a thicknessof 0.8 mm, a phenolic resin dispersion is applied in an amount of 120g/m² as a make coat precursor. Subsequently, 600 g/m² of the abrasivegrains according to the disclosure are applied by means of electrostaticscattering. After that, the make coat precursor is cured to give a makecoat. On top of the make coat and the abrasive grains, a phenolic resindispersion is applied in an amount of 800 g/m² as a size coat precursor,which is likewise cured.

FIG. C1 a shows an embodiment of an abrasive grain 10 according to thedisclosure in a perspective view. FIG. C1 b shows the first embodimentof the abrasive grain 10 according to the disclosure in a first sideview, FIG. C1 c in a second side view. FIG. C1 d shows the abrasivegrain 10 in a plan view.

The abrasive grain 10 has three main surfaces 11, 11′, 11″ and threesubsidiary surfaces 12, 12′, 12″. The subsidiary surface 12 is connectedby way of a first edge 13 to a first main surface 11 and is connected byway of a second edge 13′ to a second main surface 11′, which does notform any edge in common with the first main surface 11.

The subsidiary surface 12 includes an obtuse angle (not explicitlyrepresented in the figures) with a first main surface 11 in the regionof the first edge 13 and an obtuse angle with a second main surface 11′in the region of the second edge 13′.

By analogy, the subsidiary surface 12′ is connected by way of a firstedge to the main surface 11′ and is connected by way of a second edge tothe main surface 11″, and the subsidiary surface 12″ is connected by wayof a first edge to the main surface 11″ and is connected by way of asecond edge to the main surface 11.

The abrasive grain 10 has an abrasive grain body 14 of which the surface15 has three planar main surfaces 11, 11′ and 11″ that lie on the facesof an imaginary convex polyhedron, here a tetrahedron, the likewiseimaginary side edges of which are flatly truncated.

The abrasive grain body 14 is theoretically made up of component bodies,on the one hand the tetrahedron with truncated edges 15, on the otherhand a base 16, which has the form of a prism of which the base areacorresponds to that of the tetrahedron with the truncated edges 15.

It is of course also conceivable and within the scope of the disclosurethat, by contrast with FIGS. C1 a to C1 d, the abrasive grain body onlyconsists of a tetrahedron with truncated edges 15, but does not includea base.

With mechanical scattering onto a backing, the abrasive grain 10preferably falls on the base 16, and so the edges 13, 13′ and corners 17face away from the backing.

The abrasive grains according to the disclosure may be produced forexample by a method described hereinafter: firstly, a dispersion of 200g of α-Al₂O₃, 0.4 g of MgO, 90 g of water as a dispersion medium and 0.5g of dispersant is prepared. The MgO functions here as a nucleatingagent. The product Dolapix CE64, obtainable from Zschimmer & Schwarz,56108 Lahnstein, Germany, may be used for example as the dispersant. Thedispersion thus obtained is ground for 30 minutes at 200 revolutions perminute in a planetary ball mill, for example a planetary ball millPM400, obtainable from Retsch GmbH, 42781 Haan, Germany. Subsequently,the ground dispersion is introduced into a silicone casting moldcontaining depressions in the form of the desired abrasive grains. Forsome embodiments of the abrasive grain, an additional molding element asdescribed above may be used, for example a further casting mold, withwhich, in addition to the surface molded in the first casting mold, atleast part of the remaining surface of the abrasive grain can be molded.After that, the volatile component, that is the water, is removed fromthe dispersion. This produces an abrasive grain precursor, which isremoved from the casting mold. In a final step, the precursor issintered as bulk material at 1550° C. for 5 minutes. The dispersant isburnt out in the course of sintering.

An abrasive article according to the disclosure may for example beproduced as follows: on a backing of vulcanized fiber with a thicknessof 0.8 mm, a phenolic resin dispersion is applied in an amount of 120g/m² as a make coat precursor. Subsequently, 600 g/m² of the abrasivegrains according to the disclosure are applied by means of electrostaticscattering. After that, the make coat precursor is cured to give a makecoat. On top of the make coat and the abrasive grains, a phenolic resindispersion is applied in an amount of 800 g/m² as a size coat precursor,which is likewise cured.

FIG. D1 a shows a perspective view of a first exemplary embodiment of anabrasive grain 10 according to the disclosure, FIG. D1 b a plan view ofthe same exemplary embodiment.

The abrasive grain 10 has the form of a cone in the sense of the generaldefinition used here and has a surface 11 with two faces 12, 12′, oneface 13 being formed as the base area 13. The contour 14 of the basearea 13 has a reflex angle 15.

FIG. D2 a shows a perspective view of a second exemplary embodiment ofan abrasive grain 20 according to the disclosure, FIG. D2 b a plan viewof the same exemplary embodiment toward the base area 23.

The abrasive grain 20 has a surface 21 with seven faces, one face beingformed as the base area 23. The contour 24 of the base area 23 has threereflex angles 25.

Conversely, the contour 24 also has three acute angles 28, which are ineach case starting points for sharp cutting edges 27.

The basic form of the abrasive body 20 is a pyramid 29, the base area 30of which is formed by a star-shaped polygon 31. On account of theconcave contour of the base area 23, the faces 22 however include at theedges 27 angles that are smaller than in the case of a tetrahedron. Thisleads to an increased cutting force of the abrasive grain.

FIG. D3 a shows a perspective view of a third exemplary embodiment of anabrasive grain 130 according to the disclosure. FIG. D3 b shows a planview and FIG. D3 c shows a side view of the same exemplary embodiment.

The contour 134 of the base area 133 comprises two segments of a circle141, which are arranged symmetrically in relation to one another. Thebase area 133 corresponds to the sectional image of a double-T anchor142, which is known as the logo of the company Robert Bosch GmbH.

The base area 133 has a contour 134 with four reflex angles 135.

The abrasive grain 130 has a body that is theoretically made up of acylinder 143 in the sense of the general definition used here, the basearea of which corresponds to the sectional image of a double-T anchor142, and a saddleback roof-shaped cap 144. In fact, this abrasive grain130 is of course likewise formed as one piece and can also be producedas one piece—for example with the aid of a casting mold with depressionsthat are complementary to the abrasive grain 130.

The sectional image of the saddleback roof, an equilateral triangle, isknown as the logo of the company sia Abrasives Industries AG.

The design of the abrasive grain 130 combines functionality andrecognizability.

The abrasive grains according to the disclosure may be produced forexample by a method described hereinafter: firstly, a dispersion of 200g of α-Al₂O₃, 0.4 g of MgO, 90 g of water as a dispersion medium and 0.5g of dispersant is prepared. The MgO functions here as a nucleatingagent. The product Dolapix CE64, obtainable from Zschimmer & Schwarz,56108 Lahnstein, Germany, may be used for example as the dispersant. Thedispersion thus obtained is ground for 30 minutes at 200 revolutions perminute in a planetary ball mill, for example a planetary ball millPM400, obtainable from Retsch GmbH, 42781 Haan, Germany. Subsequently,the ground dispersion is introduced into a silicone casting moldcontaining depressions in the form of the desired abrasive grains. Afterthat, the volatile component, that is the water, is removed from thedispersion. This produces an abrasive grain precursor, which is removedfrom the casting mold. In a final step, the precursor is sintered asbulk material at 1550° C. for 5 minutes. The dispersant is burnt out inthe course of sintering.

An abrasive article according to the disclosure may for example beproduced as follows: on a backing of vulcanized fiber with a thicknessof 0.8 mm, a phenolic resin dispersion is applied in an amount of 120g/m² as a make coat precursor. Subsequently, 600 g/m² of the abrasivegrains according to the disclosure are applied by means of electrostaticscattering. After that, the make coat precursor is cured to give a makecoat. On top of the make coat and the abrasive grains, a phenolic resindispersion is applied in an amount of 800 g/m² as a size coat precursor,which is likewise cured.

The disclosure includes the following concepts:

Concept 1. An abrasive grain (110; 210; 310; 410), having a surface withat least a first face (120; 220; 320; 420) with a first outer contour(121; 221; 321; 421) and at least a second face (125; 225; 325; 425)with a second outer contour (126; 226; 326; 426), characterized in that

a. the first outer contour (121; 221; 321; 421) does not include acorner and the second outer contour (126; 226; 326; 426) includes atleast one corner (127; 227; 327; 427).

Concept 2. The abrasive grain (110; 210; 310; 410) of concept 1,characterized in that the first face (120; 220; 320; 420) issubstantially planar.

Concept 3. The abrasive grain (110; 210; 310; 410) of either of thepreceding concepts, characterized in that the first face (120; 220; 320;420) and the second face (125; 225; 325; 425) lie opposite one anotherand are arranged in relation to one another at an angle that is lessthan 30°, preferably less than 20°, more preferably less than 10°, andis particularly preferably 0°.

Concept 4. The abrasive grain (110; 210; 310; 410) of one of thepreceding concepts, characterized in that the abrasive grain (110; 210;310; 410) extends completely between the two planes that are defined bythe first face (120; 220; 320; 420) and the second face (125; 225; 325;425).

Concept 5. The abrasive grain (110; 210; 310) of one of the precedingconcepts, characterized in that the second face (125; 225; 325) isformed by a polygon.

Concept 6. The abrasive grain (110; 210) of one of the precedingconcepts, characterized in that the perpendicular projection of thesecond face (125; 225) onto the first face (120; 220) lies completelywithin the convex envelope of the first face (120; 220), in particularwithin the first face (120; 220).

Concept 7. The abrasive grain (110; 210) of concept 6, characterized inthat the abrasive grain (110; 210) tapers along a direction (R) runningperpendicularly from the first face (120; 220).

Concept 8. The abrasive grain (210) of one of the preceding concepts,characterized in that between the first face (220) and the second face(225), a lateral area (230) is formed, the lines of intersection of thelateral area (230) with at least one sectional plane (S) that extendsperpendicularly to the first face (220) and perpendicularly to thesecond face (225), in particular with each such sectional plane (5),being at least partially, preferably completely, concave, in particularstrictly concave.

Concept 9. The abrasive grain (110; 210; 310; 410) of one of thepreceding concepts, characterized in that it contains or consists of aceramic material, in particular a polycrystalline ceramic material,preferably aluminum oxide, particularly preferably α-Al₂O₃.

Concept 10. A collective of abrasive grains (110; 210; 310; 410),characterized in that it includes at least 20% by weight, preferably atleast 50% by weight, particularly preferably at least 90% by weight, ofabrasive grains (110; 210; 310; 410) of one of the preceding concepts.

Concept 11. A method for producing at least one abrasive grain (110;210; 310; 410) or a collective of abrasive grains (110; 210; 310; 410)of one of the preceding concepts, characterized by the following steps:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain (110; 210; 310; 410).

Concept 12. A casting tool for producing at least one abrasive grain(110; 210; 310; 410) of one of concepts 1 to 9, the casting toolcomprising at least one casting mold, which has at least one depression,preferably a multiplicity of depressions, with a respective surface, thesurface being complementary to the form of at least part of the surfaceof the abrasive grain (110; 210; 310; 410).

Concept 13. An abrasive article, containing a collective of abrasivegrains (110; 210; 310; 410) of concept 10.

Concept 14. A method for producing an abrasive article of concept 13,including a step in which a collective of abrasive grains (110; 210;310; 410) of concept 10 is fixed on and/or in a substrate, in particularby means of a binder.

Concept 15. A method for grinding a surface, in particular a paintedsurface, with a grinding article of concept 13.

Concept 16. An abrasive grain (110; 210) with a surface that includes atleast one main face (111; 211) with an outer contour (112; 212) that hasat least seven corners (113; 213; 213′).

Concept 17. The abrasive grain (110) of concept 16, characterized inthat the abrasive grain (110) is formed as a cone, in particular as apyramid, with a base area (111) of which the outer contour (112) has atleast seven corners (113).

Concept 18. The abrasive grain (210) of concept 16, characterized inthat the abrasive grain (210) is formed as a frustocone, in particularas a frustopyramid, with a base area (211) of which the outer contour(212) has at least seven corners (213; 213′).

Concept 19. The abrasive grain (110; 210) of one of the precedingconcepts, characterized in that the ratio of the height (h) of theabrasive grain (110; 210) and the area diameter of the main face (111;211) lies in the range from 0.8 to 1.4, preferably from 0.9 to 1.2,particularly preferably from 0.95 to 1.05.

Concept 20. The abrasive grain (110; 210) of one of the precedingconcepts, characterized in that the base area (111; 211) is a polygonwith at least seven corners.

Concept 21. The abrasive grain (110) of concept 20, characterized inthat the polygon is convex.

Concept 22. The abrasive grain (110) of concept 21, characterized inthat the polygon is regular.

Concept 23. The abrasive grain (110; 210) of one of the precedingconcepts, characterized in that the main face (111; 211) issubstantially planar.

Concept 24. The abrasive grain (110; 210) of one of the precedingconcepts, characterized in that it contains or consists of a ceramicmaterial, in particular a polycrystalline ceramic material, preferablyaluminum oxide, particularly preferably α-Al₂O₃.

Concept 25. A collective of abrasive grains (110; 210), characterized inthat it includes at least 20% by weight, preferably at least 50% byweight, particularly preferably at least 90% by weight, of abrasivegrains (110; 210) of one of the preceding concepts.

Concept 26. A method for producing at least one abrasive grain (110;210) or a collective of abrasive grains (110; 210) of one of thepreceding concepts, characterized by the following steps:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain (110; 210).

Concept 27. A casting mold for producing at least one abrasive grain(110; 210) of one of concepts 16-24, the casting mold having at leastone depression, preferably a multiplicity of depressions, with arespective surface, the surface being complementary to the form of atleast part of the surface of the abrasive grain (110; 210).

Concept 28. An abrasive article, containing a collective of abrasivegrains (110; 210) of concept 25.

Concept 29. A method for producing an abrasive article of concept 26,including a step in which a collective of abrasive grains (110; 210) ofconcept 25 is fixed on and/or in a substrate, in particular by means ofa binder.

Concept 30. A method for grinding a surface, in particular a paintedsurface, with an abrasive article of concept 28.

Concept 31. An abrasive grain (10) with at least two main surfaces (11,11′, 11″) and at least one subsidiary surface (12, 12′, 12″), which isconnected by way of a first edge (13) to a first main surface (11) andis connected by way of a second edge (13′) to a second main surface(11′), which does not have any edge in common with the first mainsurface (11), characterized in that the subsidiary surface (12) includesan obtuse angle with a first main surface (11) in the region of thefirst edge (13) and an obtuse angle with a second main surface (11′) inthe region of the second edge (13′).

Concept 32. An abrasive grain (10) with a defined form, in particular anabrasive grain (10) of concept 31, characterized in that the abrasivegrain (10) has an abrasive grain body (14) of which the surface (15) hasat least two substantially planar main surfaces (11, 11′, 11″) that lieon the faces of an imaginary convex polyhedron, in particular a Platonicsolid, an Archimedean solid, a Catalan solid, a prism or antiprism, onthe faces of a linearly distorted Platonic solid, Archimedean solid,Catalan solid, prism or antiprism or on the faces of an imaginarycombination of the solids mentioned, the abrasive grain body having atleast one flatly truncated edge.

Concept 33. The abrasive grain (10) of concept 31 or 32, characterizedin that some of the faces (11, 11′, 11″) of the abrasive grain (10), inparticular main surfaces, lie on the faces of an imaginary pyramid or animaginary frustopyramid, in particular on the faces of a tetrahedron.

Concept 34. The abrasive grain (10) of one of the preceding concepts,characterized in that each main surface (11, 11′; 11″) is connected toat least one other main surface (11, 11′, 11′) by way of a subsidiarysurface (12, 12′, 12″).

Concept 35. The abrasive grain (10) of one of the preceding concepts,characterized in that it contains or consists of a ceramic material, inparticular a polycrystalline ceramic material, preferably aluminumoxide, particularly preferably α-Al₂O₃.

Concept 36. A collective of abrasive grains (10), characterized in thatit includes at least 20% by weight, preferably at least 50% by weight,particularly preferably at least 90% by weight, of abrasive grains (10)of one of the preceding concepts.

Concept 37. A method for producing at least one abrasive grain (10) or acollective of abrasive grains (10) of one of the preceding concepts,characterized by the following steps:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain (10).

Concept 38. A casting mold for producing at least one abrasive grain(10) of one of concepts 31 to 35, the casting mold having at least onedepression, preferably a multiplicity of depressions, with a respectivesurface, the surface being complementary to the form of at least part ofthe surface of the abrasive grain (10).

Concept 39. An abrasive article, containing a collective of abrasivegrains (10) of concept 36.

Concept 40. A method for producing an abrasive article of concept 39,including a step in which a collective of abrasive grains (10) ofconcept 36 is fixed on and/or in a substrate, in particular by means ofa binder.

Concept 41. A method for grinding a surface, in particular a paintedsurface, with an abrasive article of concept 39.

Concept 42. An abrasive grain (10; 20; 130) with a defined form, theabrasive grain (10; 20; 130) having a body which is simply connected andhas a surface (11; 21) with at least two faces, in particular at leastfive faces (12, 12′; 22, 22′), preferably at least seven faces, one facebeing formed as a base area (13; 23; 133), characterized in that thecontour (14; 24; 134) of the base area (13; 23; 133) has at least onereflex angle (14; 25; 135).

Concept 43. The abrasive grain (10; 20) of concept 42, characterized inthat the abrasive grain (10; 20) or a component body of the abrasivegrain (10; 20) has the form of a cone (10; 29), in particular of apyramid (29), or a frustocone, in particular a frustopyramid, the basearea (23) being formed in particular by a star-shaped base area (13; 30)of the cone (10; 29), in particular of the pyramid (29), or thefrustocone, in particular the frustopyramid.

Concept 44. The abrasive grain (130) of concept 42 or 43, characterizedin that the abrasive grain (130) or a component body of the abrasivegrain (130) has the form of a cylinder (143), in particular of a prism,or of an antiprism, the base area (23) being formed in particular by astar-shaped polygon.

Concept 45. The abrasive grain (130) of concept 42 or 44, characterizedin that the contour (134) of the base area (133) comprises at least onesegment of a circle, preferably at least two and particularly preferablyprecisely two segments of a circle (141), and in particular correspondsto the sectional image of a double-T anchor (142).

Concept 46. The abrasive grain (10; 20; 130) of one of the precedingconcepts, characterized in that it contains or consists of a ceramicmaterial, in particular a polycrystalline ceramic material, preferablyaluminum oxide, particularly preferably α-Al₂O₃.

Concept 47. A collective of abrasive grains (10; 20; 130), characterizedin that it includes at least 20% by weight, preferably at least 50% byweight, particularly preferably at least 90% by weight, of abrasivegrains (10; 20; 130) of one of the preceding concepts.

Concept 48. A method for producing at least one abrasive grain (10; 20;130) or a collective of abrasive grains (10; 20; 130) of one of thepreceding concepts, characterized by the following steps:

a. preparing or providing a dispersion, containing α-alumina particlesand/or particles that can be converted into α-alumina, and also at leastone volatile dispersion medium, preferably water;

b. introducing the dispersion into at least one depression in a castingmold;

c. optionally wiping an upper side of the casting mold in order toremove at least part of the dispersion that stands above the upper sideof the casting mold;

d. removing part of the volatile components of the dispersion, so as toform at least one abrasive grain precursor;

e. removing the abrasive grain precursor from the casting mold;

f. optionally calcining the abrasive grain precursor;

g. sintering the abrasive grain precursor in order to obtain at leastone abrasive grain (10; 20; 130).

Concept 49. A casting tool for producing at least one abrasive grain(10; 20; 130) of one of concepts 42 to 46, the casting tool comprisingat least one casting mold, which has at least one depression, preferablya multiplicity of depressions, with a respective surface, the surfacebeing complementary to the form of at least part of the surface of theabrasive grain (10; 20; 130).

Concept 50. An abrasive article, containing a collective of abrasivegrains (10; 20; 130) of concept 47.

Concept 51. A method for producing an abrasive article of concept 50,including a step in which a collective of abrasive grains (10; 20; 130)of concept 47 is fixed on and/or in a substrate, in particular by meansof a binder.

Concept 52. A method for grinding a surface, in particular a paintedsurface, with a grinding article of concept 50.

1. An abrasive grain comprising: at least two main surfaces; and atleast one subsidiary surface, which is connected by way of a first edgeto a first main surface of the at least two main surfaces and isconnected by way of a second edge to a second main surface of the atleast two main surfaces, wherein the second main surface does not haveany edge in common with the first main surface, and wherein the at leastone subsidiary surface includes a first obtuse angle with the first mainsurface in a region of the first edge and a second obtuse angle with thesecond main surface in a region of the second edge.
 2. The abrasivegrain as claimed in claim 1, wherein the abrasive grain has a definedform, and wherein the abrasive grain has an abrasive grain body whichhas the at least two main surfaces, the at least two main surfaces beingsubstantially planar and lying on faces of (i) an imaginary convexpolyhedron, which includes a Platonic solid, an Archimedean solid, aCatalan solid, a prism or antiprism, (ii) on faces of a linearlydistorted Platonic solid, Archimedean solid, Catalan solid, prism orantiprism, or (iii) on faces of an imaginary combination of a pluralityof a Platonic solid imaginary convex polyhedron, an Archimedean solidimaginary convex polyhedron, a Catalan solid imaginary convexpolyhedron, a prism imaginary convex polyhedron, an antiprism imaginaryconvex polyhedron, a linearly distorted Platonic solid, a linearlydistorted Archimedean solid, a linearly distorted Catalan solid, alinearly distorted prism, and a linearly distorted antiprism, andwherein the abrasive grain body has at least one flatly truncated edge.3. The abrasive grain as claimed in claim 1, wherein at least one of theat least two main surfaces of the abrasive grain lies on faces of animaginary pyramid, an imaginary frustopyramid, or a tetrahedron.
 4. Theabrasive grain as claimed in claim 1, wherein each of the at least twomain surfaces is connected to at least one other main surface of the atleast two main surfaces by way of a subsidiary surface of the at leastone subsidiary surface.
 5. The abrasive grain as claimed in claim 1,wherein the abrasive grain includes a ceramic material.
 6. The abrasivegrain as claimed in claim 5, wherein the ceramic material is apolycrystalline ceramic material
 7. The abrasive grain as claimed inclaim 6, wherein the polycrystalline ceramic material includes aluminumoxide.
 8. The abrasive grain as claimed in claim 7, wherein thealuminium oxide includes α-Al₂O₃
 9. A casting mold for producing theabrasive grain of claim 1, the casting mold comprising: at least onedepression, each depression of the at least one depression including arespective surface that is complementary to a form of at least part ofat least one of (i) at least one of the at least two main surfaces and(ii) the subsidiary surface of the abrasive grain.
 10. A collective ofabrasive grains, comprising: a plurality of first abrasive grains, eachfirst abrasive grain of the plurality of first abrasive grainscomprising: at least two main surfaces; and at least one subsidiarysurface, which is connected by way of a first edge to a first mainsurface of the at least two main surfaces and is connected by way of asecond edge to a second main surface of the at least two main surfaces,wherein the second main surface does not have any edge in common withthe first main surface, wherein the at least one subsidiary surfaceincludes a first obtuse angle with the first main surface in a region ofthe first edge and a second obtuse angle with the second main surface ina region of the second edge, and wherein the collective includes atleast 20% by weight of the first abrasive grains.
 11. The collective ofabrasive grains as claimed in claim 10, wherein the collective includesat least 50% by weight of the first abrasive grain.
 12. The collectiveof abrasive grains as claimed in claim 10, wherein the collectiveincludes at least 90% by weight of the first abrasive grain.
 13. Anabrasive article comprising: the collective of abrasive grains asclaimed in claim
 10. 14. A method for producing the abrasive article asclaimed in claim 13, comprising: fixing the collective of abrasivegrains on and/or in a substrate with a binder.
 15. A method for grindinga surface or a painted surface, comprising: grinding the surface or thepainted surface with the abrasive article of claim
 13. 16. A method forproducing at least one abrasive grain or a collective of abrasive grainscomprising: preparing or providing a dispersion containing (i) at leastone of α-alumina particles and particles that can be converted intoα-alumina, and (ii) at least one volatile dispersion medium; introducingthe dispersion into at least one depression in a casting mold; removingpart of the volatile dispersion medium of the dispersion, so as to format least one abrasive grain precursor; removing the at least oneabrasive grain precursor from the casting mold; sintering the at leastone abrasive grain precursor in order to obtain the at least oneabrasive grain.
 17. The method as claimed in claim 16, wherein the atleast one volatile dispersion medium includes water.
 18. The method asclaimed in claim 16, further comprising: wiping an upper side of thecasting mold in order to remove at least part of the dispersion thatstands above the upper side of the casting mold.
 19. The method asclaimed in claim 16, further comprising: calcining the at least oneabrasive grain precursor.